Level 2 Science Notes: Application 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) describe a force as a pull or a push. (b) show an understa...

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