Level 2 Notes 2024 - 01Application of Forces.pdf

Catholic High School Level 2 Lower Secondary Science Topic 1: Application of Forces and Transfer of Energy Learning Outcomes Students should be able to: (a) descri...

Catholic High School Level 2 Lower Secondary Science Topic 1: Application of Forces and Transfer of Energy Learning Outcomes Students should be able to: (a) describe a force as a pull or a push. (b) show an understanding that a force can be a contact or non-contact force, e.g., friction (contact force); magnetic force, gravitational force (non-contact force). (c) measure force, using newton (N) as the SI unit. (d) understand that a gravitational field is the region in which a mass experiences a force due to gravitational attraction. (e) define gravitational field strength g as gravitational force per unit mass. (f) recall and apply the relationship weight = mass  gravitational field strength to new situations or to solve related problems. (g) compare weight and mass. (h) show an understanding that friction is a contact force that opposes motion or tendency for motion. (i) state the good and bad effects of friction and means to reduce friction. (j) list some everyday life examples of application of forces. (k) show curiosity about the destructive power of forces in nature (e.g., earthquakes, tsunamis, volcanic eruptions, tropical cyclones). (l) recognise that the interactions between two or more objects, result in a transfer of energy which can/may cause changes (by application of force) to the - state of rest or motion of an object - size and/or shape of an object - turning effects in objects (e.g., spanners, levers to open tins) - pressure on objects (m) describe and predict the effects of forces on: the state of rest or motion of a body, the size and shape of a body (n) describe the effect of balanced and unbalanced forces on a body. (o) identify forces acting on an object and draw free body diagram(s) representing the forces acting on the object (for cases involving forces acting in at most 2 dimensions). (p) state and understand Newton’s Three Laws of Motion [for IP only] (q) investigate pressure using the formula, pressure = force/area. (r) show an appreciation of some daily life phenomena associated with pressure (e.g., high-heeled shoes, cutting edge of a knife), atmospheric pressure (e.g., use of suction cups, drinking from straws) and pressure due to liquids (e.g., submarines have depth limits). (s) explain the meaning of energy and state the SI unit of work and energy as the joule. (t) show an understanding that there are different energy stores, e.g. kinetic, potential (gravitational, chemical, elastic), nuclear and internal, and that energy can be transferred from one store to another: a. Mechanically (by a force acting over a distance) b. Electrically (by an electric current) c. By heating (due to a temperature difference) d. By propagation of waves (both electromagnetic and mechanical) (u) identify that work done is an example of energy transfer that occurs when an object moves in the direction of a force. (v) recognise that energy cannot be created nor destroyed, but can be transferred from one store to another. (w) show an appreciation of the uses of various sources of energy (e.g., fossil fuels, solar, hydro-electric, wind energy, geothermal energy, biofuels and nuclear energy) and their impact on the environment. Last reviewed on 31 Dec 2022 Page 1 1. Application of Forces 1.1 Introduction What is a force? It is a push or pull that - changes the shape and / or size of an object - stops or moves an object - changes the direction of a moving object - slows down or speeds up a moving object The SI unit for force is newton (N). Examples of forces in everyday situations Fig. 1.1(a) Lifting weights Fig. 1.1(b) A flag being blown by the force of Fig. 1.1(c) Kicking a soccer ball the wind Fig. 1.1(d) Opening a door by pushing Fig. 1.1(e) A car engine driving the car Fig. 1.1(f) Jet engines propelling an or pulling it forward airplane forward Fig. 1.1(g) Destructive forces of Fig. 1.1(h) Tsunamis are waves caused by Fig. 1.1(i) Tropical cyclones may earthquakes sudden movement of the ocean surface due damage installations, dwellings, and to earthquakes. communication systems, resulting in loss of lives and property. Last reviewed on 31 Dec 2022 Page 2 Contact and non-contact forces Contact forces are forces that act between two objects that are physically touching each other. Non-contact forces are forces that act between two objects that are not physically touching each other. Table 1.1 Forces can be classified into contact and non-contact forces. Types of forces Quantities that Contact Non-contact are not forces friction (e.g. air gravitational mass resistance) elastic magnetic pressure tension electrostatic energy applied work normal contact power Examples of Contact Forces normal contact Fig. 1.2(a) Friction between the tyres and Fig. 1.2(b) The tension in the rope pulls Fig. 1.2(c) Normal contact force is the the road surface the glider forward. push exerted by a surface on an object pressing on it. Non-contact forces Fig. 1.2(d) Magnetic force is the force Fig. 1.2(e) Tides are caused by the Fig. 1.2(f) Static electric charges can exerted by one magnet to another moon’s gravitational force pulling on the attract and repel other static charges magnet and magnetic materials such as ocean waters. nearby. iron and steel. 1.2 Measuring forces Forces are measured with force-meters or Newton meters (e.g. spring balances). Does a weighing machine measure the mass or weight of an individual? Fig. 1.3 Types of spring balances Last reviewed on 31 Dec 2022 Page 3 1.3 Representing forces Forces acting on an object can be represented using a simple “free-body diagram”, which can help to identify and visualise the forces and their effects. A force is represented by an arrow drawn directly from the point of action. The direction of the arrow indicates the direction of the force. The length of the arrow represents the magnitude of the force.  Try it out! #1 A box is being pulled to the right along a frictionless floor. Draw and label all the forces acting on the box. Normal contact force will be explained in more detail in Section 1.5.1. 1.4 Effects of forces 1.5 Types of forces 1.5.1 Normal contact force A force is represented by an arrow drawn directly from the point of action. The normal contact force is the force exerted on an object due to its contact with another object. Fig. 1.4 Normal contact force by table on book For example, if a book is resting upon a table, then the table is exerting an upward force on the book in order to support the weight of the book. The direction of the force is always normal (perpendicular) to and away from the surface. Last reviewed on 31 Dec 2022 Page 4 1.5.2 Gravitational force (Recap from Level 1) The weight of an object is the gravitational force exerted by Earth on the object. SI unit for weight is newton (N). Gravitational field A gravitational field is a region in which a mass experiences a force due to gravitational attraction. When a mass is placed in a gravitational field, it experiences a gravitational force i.e. weight. Fig. 1.5 Gravitational force is an attractive force and is always directed towards the centre of the Earth. The gravitational field is strongest at the surface of Earth and gets weaker farther away. In other words, the gravitational force experienced by the object due to the Earth’s gravitational field gets stronger as the object moves closer to Earth. Gravitational field strength Gravitational field strength (g) is defined as the gravitational force acting per unit mass. It tells us how strong the gravitational field is. On Earth, the gravitational field strength (g) is approximately 10 N kg–1 as compared to that of Moon which is 1.6 N kg–1. How are mass and weight related? Since weight is the gravitational force exerted on a mass within the gravitational field, it is given by weight = mass  gravitational field strength W = mg where W is measured in N m is measured in kg g is the gravitational field strength and is given as 10 N/kg on earth. Note: Mass and weight must not be used interchangeably!! Do you know? What do common weighing instruments measure? Common weighing instruments such as the bathroom scale actually measure the weight of an object and not its mass. Since they are calibrated according to Earth’s gravitational field strength, they cannot be used in places with different gravitational field strengths. Last reviewed on 31 Dec 2022 Page 5   Try it out! #2  “When I was on Earth, I weighed 60 kg. Upon reaching the Moon, I weighed 10 kg. My mass has changed!!!” State and explain whether you agree with these statements. Why do astronauts float in the International Space Station (ISS)? The sensation of weightlessness happens when the effects of gravity are not felt. Gravity exists everywhere in the universe because it is defined as the force that attracts two bodies to each other due to their masses. The International Space Station, for example, is in perpetual free fall above the Earth. Its forward motion, however, just about equals the speed of its "fall" toward the planet. This means that the astronauts inside are not pulled in any particular direction, so they float. What is the impact on the human body when it experiences weightlessness for an extended period of time? Effects of gravity on the human body Last reviewed on 31 Dec 2022 Page 6 1.5.3 Friction What is friction? a contact force that opposes motion or the tendency for motion acts in a direction opposite to the object’s direction of motion or intended motion heats up the two surfaces in contact  Try it out! #3  State the direction of friction if a person is walking to the left.. Friction also occurs in fluids (gases and liquids). This type of friction is called air or water resistance. Fig. 1.6 Objects fall at different speeds in air but fall at the same speed in a vacuum. Fig. 1.7 The streamlined helmet and crouching low posture of Fig. 1.8 The parachute increases air resistance and helps the cyclist reduces air resistance. slow the spacecraft down during entry, descent, and landing. Last reviewed on 31 Dec 2022 Page 7 Why is there friction? Although two objects might look smooth, microscopically, they are rough and jagged. As the two surfaces slide against each other, their contact is anything BUT smooth. They both grind and drag against each other, creating friction. Fig. 1.9 Irregularities between the two surfaces at the microscopic level Useful effects of friction Fig. 1.10(a) The large air resistance created by the canopy of an open Fig. 1.10(b) The graphite pencil lead marks Fig. 1.10(c) Friction between the parachute slows down a descending a mark on the paper using friction. wheel and the brakes slows down the parachutist, so that he may land safely. bicycle. Fig. 1.10(d) Friction holds a nail in place Fig. 1.10(e) Aeroplanes depend on air Fig. 1.10(f) Friction is needed in a wall. resistance to keep them up in the air. between our feet and the ground to provide the grip needed. Last reviewed on 31 Dec 2022 Page 8  Negative effects of friction Fig. 1.11(a) Friction wears out the Fig. 1.11(b) Engines, axles and other Fig. 1.11(c) An infra-red image of the rubber on the tyres. moving parts lose energy due to friction. underside of Columbia that shows heat generation during its re-entry. Negative effects of friction can be harnessed such that it can be used positively Example where this is Example where this is not Results of friction useful useful slows down objects brakes: stopping a vehicle air resistance slows down in motion cars heats up objects warming hands by rubbing drilling through metal them against each other causes drill to heat up when they are cold wears down objects polishing a metal using tyres need to be replaced that are moving sandpaper against each other Ways to reduce friction Fig. 1.12(a) Using lubricants Fig. 1.12(b) Smooth or polished surface Fig. 1.12(c) A hovercraft uses a layer of of playground slide gives the child a air to move about easily. smooth ride. Fig. 1.12(d) Racing cars have Fig. 1.12(e) Streamlined shape reduces Fig. 1.12(f) Conveyor belt uses wheels streamlined bodies to improve fluid resistance of fishes during or rollers to reduce friction so that performance. movement. goods can be moved easily. The circular shape of wheels reduces the area of the contact surfaces. Last reviewed on 31 Dec 2022 Page 9   Try it out! #4  A box is stationary on a gentle slope. Draw and label all the forces acting on the box. Forces on a plane Lift is a mechanical force generated by a solid object moving through a fluid. Thrust is a mechanical force generated by the engines to move the airplane through the air. Drag is a mechanical force generated by a solid object moving through a fluid, in opposite direction to the motion. Weight is a force caused by the gravitational attraction of the Earth. 1.6 Balanced and unbalanced forces Two persons push a stationary block of wood. Fig. 1.13 Scenario 1 State how the wood will move if both persons are pushing with the same force. It will not move / It will remain stationary. Scenario 2 State what will happen if the person on the left is pushing harder than the one on the right. It will move to the right. Last reviewed on 31 Dec 2022 Page 10 Hence, we can deduce the following from the above scenarios: 1. When the forces acting on a stationary object are balanced, a stationary object will remain at rest. 2. When the forces acting on a stationary object are unbalanced, a stationary object will move in the direction of the resultant force.. Resultant Force The resultant (net) force is the single force that has the same effect as two or more forces acting together. Two forces acting in the same direction: resultant force of two forces calculated by adding both magnitudes Two forces acting in the opposite direction: resultant force of two forces calculated by subtracting the magnitude of the smaller force from the magnitude of the larger force.  Try it out! #5  Calculate the resultant force acting on the object for each instance. 4N 4N 2N 5N 8N 6N 2N 4N 4N 5N Last reviewed on 31 Dec 2022 Page 11 Example of balanced and unbalanced forces Balanced Forces Lift = Weight, Thrust = Drag Unbalanced Forces Lift > Weight: airplane rises Weight > Lift: airplane falls Drag > Thrust: airplane slows down Thrust > Drag: airplane speeds up Fig. 1.14(a) Forces on a plane (A) Clockwise (B) Counter- The drone is a quadcopter which has 4 rotors that are clockwise all connected to individual motors, allowing them to move at different speeds. FRONT Balanced Forces Thrust = Weight: drone hovers Unbalanced Forces Thrust > Weight: drone ascends LEFT ©CHS RIGHT Weight > Thrust: drone descends When the rotors spin fast, the drone ascends, and when the rotors slow down, the drone will descend. BACK (B) Counter- (A) Clockwise clockwise Fig. 1.14(b) Forces on a drone Last reviewed on 31 Dec 2022 Page 12 1.6.1 Newton’s Laws of Motion [for IP only] In a system of more than one force, the forces can either table exerts an upward force F be balanced or unbalanced. (normal contact) on book Balanced forces If the resultant force acting on an object is zero, we say the forces acting on the object are balanced. weight of book W Fig. 1.15 The two forces (F and W) are equal in magnitude but act in opposite directions. The resultant force is zero and the book remains stationary. A force F is applied on a book so the book moves in a straight line at constant velocity book sliding across a surface at a constant velocity across a rough table. F is equal to frictional force f. F f Fig. 1.16 The two forces (F and f) are equal in magnitude but act in opposite directions. The resultant force is zero and the book moves at a constant velocity. Newton’s Laws of Motion might be counter-intuitive to your existing knowledge of forces arising from your everyday experiences. Hence, it is essential to reconcile these scientific concepts with your prior understanding of forces.  Try it out! #6  A parachutist jumps off a plane. As he jumps off the plane, he encounters air resistance throughout his motion. Describe the forces he experiences throughout the jump till he opens his parachute. Parachutist just jumps off from plane: Parachutist falling in air: When parachute opens: Last reviewed on 31 Dec 2022 Page 13 Newton’s First Law Every object will continue in its state of rest or uniform motion in a straight line, unless a resultant force acts on it. It is also known as the Law of Inertia. Galileo Galilei first wrote about this concept stating: “A body moving on a level surface will continue in the same direction at a constant speed unless disturbed.” What is inertia? Inertia is the reluctance of an object to change its state of motion. All objects resist changes in their state of motion – they tend to “keep on doing what they are doing.” Mass is a measure of how difficult it is to change an object’s motion and direction. The larger the mass, the greater its inertia. That means it is more difficult for the object to move when it is at rest, or to stop when it is in motion. Why does the Earth keep spinning? The Earth will keep spinning forever unless we add or remove energy away from it. For billions of years, the Earth’s rotation has been gradually slowing down. Estimates suggest that the length of a day currently increases by about 1.8 milliseconds every century. What do you think are some possible reasons that could be slowing the Earth down? Gravitational forces of planets e.g. Moon Seismic activities e.g. earthquakes Fig. 1.17 The continuous spinning of the Earth https://hpiers.obspm.fr/eop-pc/index.php can be explained by Newton’s first law of motion. Do you know? What is speed, velocity and acceleration? Recall that speed is the distance moved per unit time and velocity is the rate of change of displacement. Acceleration is the rate of change of velocity. Speed is a scalar quantity but both velocity and acceleration are vector quantities. N Speed: 5 m s–1 Direction : East B C Velocity: 5 m s–1 east A A B C Change in speed Change in direction Change in speed and direction Speed : 10 m s–1 Speed: 5 m s–1 Speed: 10 m s–1 Direction: East Direction: North Direction: North Velocity: 10 m s–1 east Velocity: 5 m s–1 north Velocity: 10 m s–1 north An object accelerates when its velocity changes. This means that either its speed, direction or both change. Last reviewed on 31 Dec 2022 Page 14 States of motion forces are balanced (net force = 0 N) acceleration a = 0 m/s2 object at rest object in motion in a (velocity, v = 0 m/s) straight line (velocity, v ≠ 0 m/s) stays at rest stays in uniform motion in a straight line Newton’s Second Law What happens when the resultant force on an object is not zero? When a resultant force acts on an object of constant mass, the object will accelerate in the direction of the resultant force. The product of its mass and acceleration is equal to the resultant force. Resultant force = mass  acceleration Fnet = ma Note: No calculations are required for Level 2 Science.  Try it out! #7  Calculate the resultant force acting on the object. Does the object experience an acceleration? 4N 4N 2N 5N 8N 6N Last reviewed on 31 Dec 2022 Page 15  2N 4N 4N 5N Newton’s Third Law For every action force, there is an equal and opposite reaction force, and these two forces act on mutually opposite bodies. From the above, what can you deduce about the pair of forces that forms an action-reaction pair? 1. They are opposite in direction. 2. They are equal in magnitude. 3. They act on different bodies. 4. They are the same type of forces. Remember Different, Opposite, Equal Same Examples of action and reaction pairs If body A exerts a force on body B, then body B will exert an equal but opposite force on A. Last reviewed on 31 Dec 2022 Page 16  FTB FbB FBb FBT FBb – force by Bat on FTB – normal contact force by table on FEB – gravitational force by Earth on ball book book FbB – force by ball on FBT – normal contact force by book on FBE – gravitational force by book on Bat table Earth FOW – force by octopus on water FWO – force by water on octopus FWO Fig. 1.18 An alarmed octopus may shoot swiftly backwards FOW by ejecting a jet of water from the siphon.  Try it out! #8  1. Which is an action-reaction pair? Why? normal contact force normal contact force on book by table on book by table weight of book normal contact force on table by book Last reviewed on 31 Dec 2022 Page 17  2. A book is at rest on a tabletop. Draw and label all forces acting on the table. 3. State whether each statement is true or false. (a) A moving object with no unbalanced forces acting on it will naturally come to rest. (b) An unbalanced force causes an object to move with a constant velocity. (c) When a mosquito collides with a van windscreen, the van exerts a larger force on the mosquito than the mosquito on the van. The Science of PET rockets The water rocket uses high pressure to force a fluid (e.g. water) through a restricted opening at a high velocity and this creates a force that propels the rocket in the opposite direction from the expelling fluid. Fig. 1.19 Bottle rockets are excellent devices for investigating “Newton’s Three Laws of Motion“. The current record for the greatest altitude achieved by a water and air propelled rocket is 830 metres! How Do Rockets Take Off? Rockets move upward by firing hot exhaust gas downward. This is an example of “action and reaction” forces (Newton’s third law of motion): the hot exhaust gas firing down (the action force) creates an equal and opposite force (the reaction force) that speeds the rocket up. The action force is the force of the gas, while the reaction force is the force acting on the rocket. These two forces are of equal size, but point in opposite directions and act on different objects (which is why they do not cancel out). Last reviewed on 31 Dec 2022 Page 18 1.7 Pressure The effects of exerting a force on varying areas can be very different. For example, pressing on the flat end of a thumbtack fulfils its intended purpose, but pressing on the sharp end can cause injury instead. This effect is referred to as pressure, and is determined by the ratio of the force applied to the area on which it is applied. Fig. 1.20 Pressing on the different ends of a thumbtack www.s-cool.co.uk/gcse/physics/forces-moments-and-pressure/revise-it/forces-and-pressure Pressure is defined as force acting per unit area F p= A (for solids, liquids and gases) where: p = pressure acting on surface (SI unit: N/m2 or pascal, Pa), F = force acting normally (i.e. perpendicularly) on surface (N), A = area of surface on which force is acting (m2). 1 Pa is equivalent to 1 N/m2.  Try it out! #9  Let’s consider two different scenarios: Scenario 1: A force of 10 N is applied to a balloon on a nail of surface area 0.1 cm2. Scenario 2: A force of 10 N is applied to a similar balloon on a bed of nails. 50 nails are in contact with the balloon. The surface area of each nail is 0.1 cm2. 10 N 10 N ©CHS ©CHS Scenario 1: A balloon pressed onto 1 nail Scenario 2: A balloon pressed onto 50 nails Calculate the pressure (leave your answers in N/cm2) exerted on the balloon in each scenario. Comment on which balloon is more likely to burst. Scenario 1: Scenario 2: Last reviewed on 31 Dec 2022 Page 19 1.7.1 Applications of pressure Fig. 1.21(a) Cutting edges of knives have very small areas. A Fig. 1.21(b) A large pressure is created at the sharp point end small force produces a large pressure on the cutting edges. of a pin which allows it to pierce paper and wooden boards. Fig. 1.21(c) The spikes on the soles of football shoes have Fig. 1.21(d) Stiletto heels often leave unsightly marks in small area. The large pressure produced by the spikes carpets. This is because the weight of the wearer exerts a large increases the shoes’ grip on the ground. pressure on a small area of ground. Fig. 1.21(e) Broad handles of luggage are provided for Fig. 1.21(f) Vehicles with heavy loads have more than four comfort so that the pressure exerted on the traveller’s hands tyres to reduce pressure on each tyre. are small. Can you think of other examples? Last reviewed on 31 Dec 2022 Page 20  1.7.2 Pressure in air – Atmospheric pressure The air around us exerts pressure on all objects that are exposed to air. The pressure exerted by the air in the Earth’s atmosphere is called atmospheric pressure. At sea level, it is 1 atmosphere (1 atm), which is about 100 000 N/m² or 1 x 105 Pa. We are not crushed by this pressure because the internal pressure within our bodies is about the same as well. Atmospheric pressure decreases with increasing height. At sea level, the atmospheric pressure is about 1 x 105 Pa. sea level ©CHS Pressure increases with increasing depth. Fig. 1.22 Relationship between pressure, height above sea level and depth below sea level Applications of atmospheric pressure By sucking through the straw, we drinking lower the air pressure in the straw straw. The higher atmospheric pressure outside the straw pushes the liquid up the drinking straw. liquid Fig. 1.23(a) Atmospheric pressure acting on the rubber Fig. 1.23(b) Drinking water using a straw sucker holds it tightly on the wall. Last reviewed on 31 Dec 2022 Page 21 1.7.3 Pressure in liquids When we are underwater, we feel the pressure of water on our eardrums. The Earth’s gravitational pull acts on all objects, including liquids. This causes liquids to have weight. A body of liquid (e.g. a pool of water) exerts pressure on an object (e.g. our eardrums) placed in it because of its weight. At greater depths, the weight of the liquid above a body submerged in the liquid is greater. Therefore, the pressure is greater. Water spurts farther and faster away from outlet 3 than outlet 2, and from outlet 2 h1 outlet 1 than outlet 1. h2 h3 This shows that liquid pressure increases outlet 2 with depth. outlet 3 Fig. 1.24 Pressure in liquids at different depths Application of pressure in liquid The deeper the submarine dives, the greater the underwater pressure sea submarine becomes. Fig. 1.25 Submarines dive to great depths underwater. Their rigid metal bodies are built to withstand the very high pressure deep underwater. Design of spacesuits The space suit provides air pressure to keep the fluids in your body in a liquid state, preventing it from boiling. Like a tyre, a space suit is essentially an inflated balloon that is restricted by some rubberised fabric, in this case, Neoprene-coated fibres. The restriction placed on the "balloon" portion of the suit supplies air pressure on the astronaut inside, like blowing up a balloon inside a cardboard tube. Most space suits operate at pressures below normal atmospheric pressure; the space shuttle cabin also operates at normal atmospheric pressure. The space suit used by shuttle astronauts operates at 0.29 atm. Therefore, the cabin pressure of either the shuttle itself or an airlock must be reduced before an astronaut gets suited up for a spacewalk. A spacewalking astronaut runs the risk of getting the decompression sickness because of the changes in pressure between the space suit and the shuttle cabin. Last reviewed on 31 Dec 2022 Page 22 1.8 Turning Effect of Force A force can produce a turning effect, which is called the moment of a force. The moment of a force about a pivot is the product of the magnitude of the force and the perpendicular distance of the line of action of the force from the pivot. line of action of F F pivot d Fig. 1.26 Moment of a force = force (in N) x perpendicular distance of line of action of force from the pivot (in m) The SI unit for a moment is newton-metre (Nm), with the direction of the moment stated as either Anti-clockwise or Clockwise. The point about which a force turns is called the pivot. Can you identify the pivots in the following examples? Fig. 1.27(a) Steering wheel Fig. 1.27(b) Seesaw Fig. 1.27(c) Canoeing 1.8.1 Applications of moment of a force force turning effect turning effect force Fig. 1.28(a) Crane remains stout despite lifting masses Fig. 1.28(b) Pulling a nail out of a wooden block is made easier weighing tons of kilograms. with the help of the claw side on a hammer. Last reviewed on 31 Dec 2022 Page 23  turning effect force force turning effect Fig. 1.28(c) When we open the door, we apply a force on the Fig. 1.28(d) A trebuchet flings heavy stones through the air by door knob or handle. hanging something heavier on the other side of a pivoted arm. force force turning effect turning effect Fig. 1.28(e) A person applies an upward force on the handles Fig. 1.28(f) A person applies a large force on the fishing rod of a wheelbarrow. The heavy load becomes easier to move. handle to lift the fishing rod. The fish that is caught at the other end of the rod moves over a large distance. 1.8.2 Factors affecting moment of a force The turning effect of a force depends on: 1. size of the force 2. perpendicular distance of the line of action of force from the pivot Fig. 1.29 When the hand is far away from the pivot, a small force is required. When the hand is near to the pivot, a large force is required. Last reviewed on 31 Dec 2022 Page 24  1. Size of the force A screwdriver can be used to open the lid of a can. If the force is applied at the same position, the greater the applied force, the greater the turning effect of the force. d 2. Perpendicular distance from the pivot F The force, F, is applied near the pivot. The perpendicular distance, d, from the line of action of force to the pivot is small. line action of force pivot The turning effect of the force is small. Fig. 1.30 The force, F, is applied near the pivot. The same force, F, is applied further away from d the pivot. The perpendicular distance, d, from the F line of action to the pivot is large. The turning effect of the force is large. line action pivot of force Fig. 1.31 The force, F, is applied further away from the pivot.  Try it out! #10  1. The spanner is used to tighten a nut. X Y 5.0 cm 15.0 cm Calculate the moment created about the nut if a force of 20 N is applied downwards, at (a) X (b) Y Last reviewed on 31 Dec 2022 Page 25  (c) The same spanner is now used to loosen a nut. A clockwise moment of 5.0 N m is needed to loosen the nut. X Y 5.0 cm 15.0 cm Calculate the minimum force required to loosen the nut at (i) X (ii) Y 2. In the diagram on the right, a boy exerts a force of 400 N vertically downwards at one end of a lever in order to lift a boulder off the ground. Calculate the turning moment created by the boy about the pivot. 0.75 m Moment created about the pivot = F × d = 400 × 0.60 = 240 N m Anti-clockwise 0.60 m 1.8.3 Principle of moments Why does a beam balance? This is because the clockwise moment generated by the weight of the mass is equal to the anti- clockwise moment generated by the weight of the apple! Fig. 1.32 Why does a beam balance? clockwise moment = anti-clockwise moment Sg x d = mg x d The Principle of Moments states: When an object is in equilibrium, the sum of clockwise moments about any point is equal to the sum of anti-clockwise moments about the same point. Last reviewed on 31 Dec 2022 Page 26   Try it out! #11  A metre rule is supported at its centre. It is balanced by 2 weights, A and B, as shown in the figure. If the weights of A and B are 40 N and 20 N respectively, find the distance of weight B from the support. Last reviewed on 31 Dec 2022 Page 27 2 Transfer of Energy Energy is the ability to do work. Both living and non-living systems need energy to function. Unlike matter, energy does not occupy space and has no mass. The SI unit for energy is Joule (J). 2.1 Categories of energy We get energy from many sources: renewable and non-renewable. Renewable Fig. 1.33(a) Biofuels are derived from animal and plant matter Fig. 1.33(b) Geothermal energy is derived from hot rocks deep such as water hyacinth and sugar cane. underground in volcanic areas. By drilling deep into earth, water flowing through huge underground pipes is heated into steam. Fig. 1.33(c) A hydroelectric power station stores water in a Fig. 1.33(d) The energy in sunlight can be directly transferred reservoir behind a dam. The flow of water from the reservoir electrically by photovoltaic or solar cells to provide energy for turns the blades of a turbine to generate electricity. other purposes. Fig. 1.33(e) Wind energy is an energy source that converts the Fig. 1.33(f) Nuclear power is the use of nuclear reactions to energy of moving air (wind) into electricity by rotating one or produce electricity. Presently, the vast majority of electricity more turbines. from nuclear power is produced by nuclear fission of uranium and plutonium. Last reviewed on 31 Dec 2022 Page 28 Non-renewable Fig. 1.33(g) Fossil fuels are formed by the remains of dead plants and animals. It takes millions of years for fossil fuels to form. Energy Stores To get a better understanding of energy stores and energy transfers, we will use the analogy of energy as money and energy stores as different bank accounts where money can be stored. The six different energy stores are shown below. Fig. 1.34 These are the six energy stores. Last reviewed on 31 Dec 2022 Page 29 2.1.1 Gravitational Potential Energy Gravitational potential energy (GPE) is the energy in the gravitational potential store of a body because of its position relative to the ground. GPE depends on the mass (m) of the object, height (h) the object is at and gravitational field strength (g) where the object is. g has a value of 10 N kg-1 on Earth. Formula (for information only) GPE = mgh Fig. 1.35 When a pile hammer is raised above the ground, it gains gravitational potential energy due to its elevated (raised) position in the air.  Try it out! #12  1. Which object possesses gravitational potential energy? A. stationary pendulum bob hanging on a taut string B. boy at the top of a slide C. wall clock affixed to the wall D. all of the above Answer: D 2.1.2 Chemical Potential Energy Energy stored in food, fossil fuels and electric cells is called chemical potential energy. Chemical potential energy is the energy which holds the atoms and molecules of substances together. This energy is transferred during chemical reactions. Fig. 1.36(a) Chemical potential energy Fig. 1.36(b) Burning charcoal causes its Fig. 1.36(c) When batteries are in food is transferred to the cell during chemical potential energy to be connected in an electrical circuit, its cellular respiration for the cell to do work. transferred to food to cook it by chemical potential energy is transferred increasing its temperature. electrically by an electric current to other components in the circuit. Last reviewed on 31 Dec 2022 Page 30 2.1.3 Elastic Potential Energy Elastic potential energy is the energy stored in an elastic object when it is pushed or pulled. The energy is stored until the force is removed and the object springs back to its original shape, doing work in the process. Fig. 1.37(a) The twisted rubber band powers a toy airplane when released. Fig. 1.37(b) An archer's stretched bow causes the arrow to Fig. 1.37(c) A bent diving board just before a diver jumps causes fly when released. the diver to increase its kinetic energy after he jumps. 2.1.4 Kinetic Energy Kinetic energy is the energy in the kinetic store of a body due to its motion. Kinetic energy is dependent on the mass (m) of the object and the speed (v) of the object. For your information: 1 KE = 2 mv2 (where m = mass of the object in kg, v = Fig. 1.38 A moving train possesses a lot of kinetic velocity of the object in m/s) energy. If 2 objects have the same mass but are moving at different speeds, the one with the greater speed will have more kinetic energy. A bullet (with a small mass) being thrown around does not kill but a speeding bullet fired out from a gun barrel can! Last reviewed on 31 Dec 2022 Page 31 2.1.5 Internal Energy Internal energy is an energy store that is made up of the total kinetic energy of the particles and total potential energy between the particles of an object. In general, when an object becomes warmer, the total kinetic energy of its particles increases, so its internal energy increases. Conversely, when an object becomes cooler, the total kinetic energy of its particles decreases, so its internal energy decreases. [For information only]: In addition, when an object changes from solid to liquid state or liquid to gaseous state without a change in temperature, the total potential energy between the particles increases, so its internal energy increases. You will learn more about potential energy between particles in Upper Secondary Physics. Fig. 1.39(a) During cooking, the internal Fig. 1.39(b) When hot water flows Fig. 1.39(c) When water is heated to energy of the food (and water) increases through the radiators, it becomes cooler become steam in power plants, its as they become hotter. when some of its internal energy is internal energy increases. The transferred to the surrounding air to pressurised steam then drives turbines warm the house. that produce electric current. 2.1.6 Nuclear Energy Nuclear energy is the energy stored in the nucleus of an atom, which holds the protons and neutrons together. This energy is released during nuclear reactions. There are two types of nuclear reactions. Nuclear fission The breaking down of a larger nucleus by bombarding the atom of a fuel (uranium) with a neutron, splitting it into two smaller nuclei and two or three neutrons to carry on the reaction with another two or three fuel atoms. Nuclear fission is used in nuclear bombs and nuclear power stations. Fig. 1.40 The fission of heavy elements releases large amounts of energy to its surroundings. Nuclear fusion The combining of two small nuclei to form a larger, unstable nucleus, giving off energy in the form of a fast- moving neutron. Nuclear fusion is found in hydrogen bombs, the Sun and other galactic stars. Fig. 1.41 Fusion can involve many different elements and produces more energy than fission. Last reviewed on 31 Dec 2022 Page 32  Nuclear energy in space exploration The independence and longevity of nuclear energy makes it far more superior than any other power source currently used in space travel. Nuclear energy could be harvested through several different methods such as nuclear reactors and generators. Common nuclear applications and technologies used in non-manned space explorations include the Radioisotope Thermoelectric Generator (RTG) which uses heat produced by radioactive material. However, for manned missions that involve greater duration, technologies harvesting nuclear fission are the preferred (and faster) means of travelling the solar system. Fig. 1.42 Mars Curiosity rover powered by a RTG on Mars. White RTG with fins is visible at the far side of the rover. What is the difference between chemical potential energy and nuclear energy? Chemical potential energy is the energy released when there is a forming of bonds between atoms and molecules. Nuclear energy is the energy released when there is a change in the atomic structure of an element (ie. breaking down of bonds between protons and neutrons).  Try it out! #13  List down one advantage and one disadvantage of using nuclear energy to generate power.  Try it out! #14  A stone is thrown towards the surface of a still lake. Just as the stone hits the surface of the water, state the main store(s) of energy the stone possesses. A maglev train levitates in air due to electromagnetic repulsion between the tracks and the train. State the main store of energy the train possesses when it is at the train station. Last reviewed on 31 Dec 2022 Page 33 2.2 Energy Transfers Continuing the analogy of energy as money, money (energy) can be transferred between bank accounts (energy stores) in different ways, for example, using Internet banking or ATM machines. Similarly, energy can be transferred to different energy stores through different ways. Fig. 1.43 The four energy transfers 2.2.1 Energy Transfer Mechanically by a Force Acting Over a Distance (Work Done) When a force is applied to an object and the object moves in the direction of the force, there is work done on the object by the force. Work done by a constant force on an object is given by the product of the force and the distance moved by the object in the direction of the force. Work done = Force x Distance moved in the direction of the force W=Fxd *Note the difference in formulae between work done and turning effect of the force. Even though the formula seems to be the same (ie. Moment = F x d), the d in the formula for moment refers to the “perpendicular distance from the pivot to the line of action of F). A box is initially stationary on the ground. Hence, it has F=1N no kinetic energy and no gravitational potential energy. When a person applying a force of 1 N moves the box 1m 1 m in the direction of the force, the work done by the force is 1 J. Fig. 1.44 Work done in moving a box in the direction of the force The SI unit for work done is Joule (J). One joule of energy is needed to do one joule of work. Assuming a frictionless ground, the energy (from the person applying the force) has been transferred mechanically to the box by the force acting over a distance. Hence, the box now has 1 J of kinetic energy. Last reviewed on 31 Dec 2022 Page 34 In order for work to be done, these conditions must be met: A force acts on an object. The object moves in the same direction as the force. No work is done: motion if a force is exerted on an object, but it does not move. if an object moves but the force exerted perpendicular is perpendicular to the direction of motion gravitational force of the object. Fig. 1.45 Gravitational forces keep satellites in orbit, but no work is done (by gravitational force) since direction of motion is perpendicular to the force. There is work done on the box No work is done on the basket. No work is done on the basket. A force of 50 N is because a force of 40 N is applied to A force of 50 N is exerted on the exerted on the basket, which moves over a move the box over a distance of 2 m. basket. However, the basket does distance. However, the direction of motion of the The box moves in the same direction not move. basket is perpendicular to the direction of the in which the force was exerted. force.  Try it out! #15  1. A crane lifted a scrapped car of mass 1500 kg, 8.0 m off the ground. Calculate the work done by the crane. 2(a). A woman pushes an empty trolley with a force of 30 N for 2.0 m. Calculate the work done by the woman. 2(b). The work done when she pushes the trolley loaded with groceries for 50 cm is the same as when she pushed the empty trolley for 2.0 m. Calculate the force that she applied to push the loaded trolley with a constant speed. Last reviewed on 31 Dec 2022 Page 35 2.2.2 Energy Transfer Electrically by an Electric Current Energy can be transferred electrically by an electric current, which is the flow of electric charge such as electrons and ions. When a battery is connected in an electric circuit, chemical potential energy in the battery is transferred electrically by an electric current to increase the energy of the other circuit components. For example, when current passes through a resistor, it heats up and hence its internal energy increases (refer to Section 2.1.5). 2.2.3 Energy Transfer by Heating due to a Temperature Difference A small block of solid butter at room temperature is placed on a pan that is on an electric stove. When the electric stove is turned on, the butter melts after some time. Fig. 1.46 The butter is heated when the electric stove is turned on When the electric stove is turned on, energy is transferred by heating from the hot surface of the pan to the butter. This transfer of energy is due to the temperature difference between the hot surface of the pan and the butter. 2.2.4 Energy Transfer by Propagation of Waves Energy can also be transferred from one object to another by propagation of waves. This could include electromagnetic waves or mechanical waves (eg sound waves, waves on a guitar string). A girl kicking a ball in an open field on a sunny day may feel warm after some time. This is because energy is transferred by propagation of electromagnetic waves from the Sun (especially infra-red waves) to increase the internal energy of the girl as she feels hotter. Fig. 1.47 A girl kicking a ball under the Sun feels hotter Last reviewed on 31 Dec 2022 Page 36 A boy hears a sound each time he hits two cymbals together. This is because energy is transferred by propagation of sound waves from the cymbals through the air to vibrate the ear drums of the boy, increasing the kinetic energy of the ear drums of the boy. Fig. 1.48 A boy hears a sound each time he hits the two cymbals together. Energy Transfers for a Roller Coaster Roller coasters As the cars reach the top of the loop, they have the maximum ____gravitational potential energy______. When the cars go down the loop, some of the _gravitational potential energy_ is transferred to _kinetic energy_. ____Internal energy____ of the cars is increased due to energy transferred mechanically by friction between the tracks and the cars, causing the cars to become hotter. __Kinetic energy__ of the ear drums of the people is increased due to energy transferred by propagation of sound waves from the cars and tracks through the air to the ear drums. 2.3 Conservation of Energy Continuing the analogy of energy as money, we can transfer money (energy) from one bank account (energy store) to another bank account (energy store) but the total amount of money in all the bank accounts collectively remains constant. The Principle of Conservation of Energy states that energy cannot be created or destroyed. It can be transferred from one store to another store. The total amount of energy before and after a transfer remains the same. Some of the energy becomes useful energy and is used to do work, while the remaining becomes wasted energy as it cannot be harnessed. Last reviewed on 31 Dec 2022 Page 37 For ease of calculation, energy lost during transfers is sometimes assumed to be negligible. Take the example of a simple pendulum oscillating: Total Mechanical Energy (ie. KE + GPE) remains the same. If GPE at top = 100 J, KE at top = 0 J (Total Mechanical Energy = 100 J) If GPE at bottom = 0 J, KE at bottom = 100 J (Total Mechanical Energy = 100 J)  Try it out! #16  A ball is dropped from a fixed height at position A. Fill in the blanks with the correct values. (a) Assume that air resistance is negligible (b) Assume that air resistance is not negligible GPE = 8000 J GPE = 8000 J A A KE = 0 J KE = 0 J GPE = 6000 J GPE = 6000 J B B Heat generated from A to B = 250 J KE = J KE = J GPE = 4000 J GPE = 4000 J C C Heat generated from A to C = 500 J KE = J KE = J GPE = 2000 J GPE = 2000 J D D Heat generated from A to D = 750 J KE = J KE = J GPE = 0 J GPE = 0 J Heat generated from A to E = 1000 J E KE = J E KE = J Last reviewed on 31 Dec 2022 Page 38 2.4 Saving Energy With rapid growth in populations, urbanisation and industrialisation, the world is consuming more energy today. In 2015, 67% of the energy sources are still from non-renewable sources (according to a report by IEA) which will run out while the cost of their use has also risen. Other than rising cost, the use of such fuels has also heightened environmental concerns regarding air and water pollution as well as global warming. Hence, there is a serious need to conserve energy and switch to the use of renewable energy sources. As a country, some possible ways to conserve energy are: Monitor closely usage of water and energy. Develop a more efficient public transport system. Manage energy usage in industries to minimise wastage and cut energy costs. Construct energy and water-efficient buildings. As an individual, we can play our part by: Taking public transport. Switching off appliances when not in use. Using fans instead of air-conditioners. Using energy-efficient appliances and lightings e.g. LED lights. Recycling where possible as producing new products from raw material uses a lot of energy. Last reviewed on 31 Dec 2022 Page 39  Fig. 1.49 National Environment Agency (NEA) green tick label Future of energy Humans continue to overcome challenges of clean-energy aviation when the Swiss Solar Impulse 2 managed to travel more than 40 000 km around the world without a single drop of liquid fuel. Across this round-the-world flight, the team of 2 pilots overcame many technical, human and operational challenges that had never been faced before. Fig. 1.50 Historic solar flight shows promise and challenge of clean energy. Last reviewed on 31 Dec 2022 Page 40