Y8 Unit 3 Forces and Energy PDF

Unit 3 forces and energy 3.1 Forces and motion 3.2 Speed 3.3 Describing movement 3.4 Turning forces 3.5 Pressure between solids 3.6 Pressure in liquids and gases 3.7 Particles on the move 3.1 Forces and motion What is a force? A f...

Unit 3 forces and energy 3.1 Forces and motion 3.2 Speed 3.3 Describing movement 3.4 Turning forces 3.5 Pressure between solids 3.6 Pressure in liquids and gases 3.7 Particles on the move 3.1 Forces and motion What is a force? A forces is a push or a pull that acts on an object due to the interaction with another object Force is measured in newtons (N) There are two main categories of forces: contact forces and non-contact forces. 3.1 Forces and motion 3.1 Forces and motion If the forces acting on the object are balanced there is no resultant force. This means the object could be travelling at a constant speed. It could also be stationary or not moving at all (stationary). Example 1: A car is travelling down a motorway. The driving force is 500 N and the friction force acting against the car is also 500 N. Resultant force = 500 N - 500 N Resultant force = 0 N There is no resultant force and the car is travelling on a motorway, this means that the car is travelling at a constant speed. 3.1 Forces and motion Example 2: A box is placed on a table. The weight of the force on the box acting downwards is 100 N. The table applies a 100 N normal reaction force on the box upwards. Resultant force = 100 N – 100 N Resultant force = 0 N There is no resultant force acting on the box and the box is at rest on the table, meaning that the box is stationary.  3.1 Forces and motion Forces acting in the same direction: In this diagram there are two forces, which, when added together give a resultant force of 30 newtons (N) Resultant force= 20N + 10N Resultant force= 30N to the right 3.1 Forces and motion Forces acting in opposite directions: In this diagram there are two forces, acting in opposite directions: Resultant force= 20N – 10N Resultant force= 10N to the left 3.1 Forces and motion Force diagram The arrows show the size and direction of each force. The longer the arrow the bigger the force. For balanced forces, the arrows are the same length but in opposite direction.  3.1 Forces and motion Forces can affect things in 3 ways: 1. A force can change the shape of an object. 2. A force can change the speed of an object, making it faster or slower. o Unbalanced forces can slow down/speed up an object. 3. A force can change the direction of an object.  3.1 Forces and motion Force of Gravity (weight): pulling it towards the center of the Earth. Contact force: occurs because of two objects contacting each other (the ground is pushing up on the rock). These two forces are balanced so the rock does not move towards the center of the Earth. If the wind starts to blow, what do you think will happen? The wind starts to blow (force). Friction exists between the rock and the ground. If the force of the wind is balanced by the friction- the stone will remain stationary. If the forces are unbalanced, then the rock will move(toward the direction of the larger force). 3.1 Forces and motion Friction: Friction is the force that opposes motion. To measure friction during an experiment we use a force meter. The kinetic energy of the moving object is converted to heat energy by the force of friction 3.1 Forces and motion Air resistance (Drag): Air resistance/drag is the force that opposes the movement of objects through air. 3.1 Forces and motion Forces when skydiving: 1. Drag (air resistance) is less than weight. o Skydiver speeds up. o Drag increases 2. Drag = Weight o Forces are balanced o Skydiver falls at a steady speed 3. Parachute opens o Drag is greater than weight o Unbalanced forces – keeps falling but slows down. 3.1 Forces and motion Changing direction When the ball contacts/hits the tennis racket, it pushes against it. The ball will go back in the opposite direction if the hitting force is larger than the force from the ball. When an object is moving in a circle, its direction is always changing. 3.1 Forces and motion Summary  3.2 SPEED q Speed=distance/time q Tells you how fast or slow an object is moving. e.g. The car’s speed was 50km/hour when it crashed. q Different units can be used in different countries e.g. km/h or m/h q Speed of ships – knots q Speed of aeroplanes - mach q Average speed is sometimes used because the speed of an object during a journey is not always constant (Constant means not changing).  3.2 SPEED REMEMBER! q The unit for speed in science is metres per second, m/s. It is not: mph, kph or mps q If the distance travelled is in km, multiply it by 1000 to get the distance in m. For example, 3.5 km = 3500 m (3.5 × 1000).  3.3 describing movement Distance-time graphs: Represent the motion of an object. It shows how the distance moved from a starting point changes over time. ✓ Distance travelled: vertical (y) axis ✓ Time taken: horizontal (x) axis ✓ The gradient of line = the speed This means that the line is: o Horizontal for a stationary object (because the distance stays the same). o A straight diagonal for an object moving at a constant (steady) speed. The steeper the line, the greater the gradient and speed. 3.3 describing movement A straight line , parallel to time axis = object is at rest STEADY SPEED 3.3 describing movement Distance/time graph 1)A – Starting position (zero distance travelled) 3) Car moves at a constant speed to C BUT faster Car travels at a constant speed from A to than it travelled between A and B. B.(travels the same distance in each second) travels a greater distance each second. Distance from A to B increases with time. Shown as a steeper, straight, upward sloping line. Shown as a straight upward sloping line. 4) C – car travels at a constant speed back to starting 2)B – car is stationary/at rest position A. only time changes; distance stays the same. Distance of the car decreases with time. Shown as a horizontal straight line. Shown as a straight, downward sloping line. 3.4 Turning forces Examples of forces that cause an object to turn. Hinges 3.4 Turning forces Moments are turning forces. - The unit for moments in science is Newton meters (Nm) Lever: the object that turns. Pivot: the point around which the lever turns. Moments | Forces & Motion | Physics | FuseSchool - YouTube 3.4 Turning forces The size of a moment depends on two things: q Size of the force that is applied. o The bigger the force, the bigger the moment. q Distance from the force to the pivot. o The bigger the distance, the bigger the moment. 3.4 Turning forces Using moments: A see-saw will balance if the moments on each side of the pivot are equal. This is why you might have to adjust your position on a see-saw if you are a different weight from the person on the other end. The principle of moments states that for an object to be balanced the total clockwise moment must be equal to the total anti-clockwise moment. Sum of Moments (anti clockwise) = Sum of Moments ( clockwise) Force x Distance = Force x Distance 350N x 2.0m = W x 1.5m 700Nm = 1.5m x W W = 700Nm/1.5m W = 467N 3.5 Pressure between solids 3.5 Pressure between solids How could you increase the pushing effect of the force? increase the force o increased force will cause an increase in pressure decrease in area o decreased area will cause an increase in pressure 3.5 Pressure between solids q As the force increases the pressure increases q As the area increases the pressure decreases 3.5 Pressure between solids 3.5 Pressure between solids 3.5 Pressure between solids q A smaller area = greater Sharp pressure with the same teeth force acting on it. q The force of their jaws creates enough pressure to crush their prey.  3.5 Pressure between solids Low pressure High pressure qDrawing pins have a large round end for your thumb to push. The round end : large area - low pressure to your thumb. qThe sharp end has a very small area. qThe same force produces a high pressure there, so it pushes into the notice board. 3.5 Pressure between solids Calculate the pressure of one foot only. F= mass x gravity Mass= 60kg/2 = 30kg F = 30kg x 10N = 300N P =F/A = 300N/0.0001 = 3000 000 N/m2 3.5 Pressure between solids Calculate the pressure of one foot only. Mass = 3000kg/4 = 750kg F=mxg = 750kg x 10 = 7500N P = F/A = 7500N/0.1m2 = 75 000N/m2 3.6 pressure in liquids and gasses Pressure exists: o on surfaces o in air o in liquids 3.6 pressure in liquids and gasses Dam wall: Increasing depth=Increasing pressure 3.6 pressure in liquids and gasses 3.6 pressure in liquids and gasses Pressure in liquid: Liquids exert pressure on objects. q The pressure in liquids changes with depth. q The deeper you go: othe greater the weight of liquid above 3.6 pressure in liquids and gasses Pressure in liquids: The weight of the liquid causes pressure in the container. It also causes pressure on any object in the liquid. Pressure acts in all directions: The liquid pushes on all surfaces it is in contact with. For a submarine this means that pressure is being exerted equally on all parts of the hull. 3.6 pressure in liquids and gasses Pressure also depends on the density of the liquid. The higher the density the higher the pressure.  3.6 pressure in liquids and gasses Atmospheric pressure: The Earth's atmosphere - outermost layer. Mixture of gases (700 km). Exerts pressure on the Earth's surface /objects 3.6 pressure in liquids and gasses Atmospheric pressure : Pressure exerted by the air. Air has mass and is attracted by Earth’s gravity, producing a force. The force exerted on a unit area of the Earth’s surface is atmospheric pressure. Atmospheric pressure = Force/Area = Mass x Gravity/ Area AREA  3.6 pressure in liquids and gasses Altitude and pressure Atmospheric pressure changes with altitude (height above sea level). The higher you go the lower the atmospheric pressure. E.g. atmospheric pressure at: Sea level = 101,325 Pa (atmospheric pressure is at its highest) Mt Everest = 33,000Pa There are two reasons why air pressure decreases as altitude increases: density and depth of the atmosphere. 3.6 pressure in liquids and gasses As the height above the surface increases: qthe number of gas molecules in a given volume of air decreases qthe frequency of collisions with objects in the air decreases qTherefore, pressure decreases  3.6 pressure in liquids and gasses The density of air decreases with height. There are two reasons: 1) At higher altitudes, there is less air pushing down from above. 2) Gravity is weaker farther from Earth's centre. Most gas molecules in the atmosphere are pulled close to Earth’s surface by gravity, so gas particles are denser near the surface. With more gas particles in a given volume there are more collisions of particles and therefore greater pressure. 3.6 pressure in liquids and gasses Each collision applies a force to the inside area of the container. Collisions of gas particles with the wall create pressure. 3.6 pressure in liquids and gasses Pressure on a ballon Pressure in Gases | Matter | Physics | FuseSchool - YouTube 3.6 pressure in liquids and gasses Effects of Atmospheric pressure: The force of the particles hitting the surface, causes pressure. 3.6 pressure in liquids and gasses The air pressure outside of the bottle is much greater than the air pressure inside the bottle so the bottle is crushed. If a bottle of air is sealed at 4,267 m above sea level and is then brought down the mountain, what do you think will happen? 3.6 pressure in liquids and gasses Pressure and temperature in gasses: qIncreasing the temperature of a gas increases the pressure of a gas. qAs the temperature increases, the gas particles gain energy and move faster. qThe collisions of the particles exert more force. 3.6 pressure in liquids and gasses Pressure and temperature in gasses: q High pressure used to cook food. q Water and food are placed inside the pot with a sealed lid. q Water boils, steam can’t escape, increasing the pressure of the gas. q This increases the temperature. When the required pressure is reached the steam escapes through a valve.  3.7 PARTICLES ON THE MOVE Mixing gasses: The particles in a gas move randomly. Speed of the particles and direction of the particles are also random. If we mix two gases in one container, each one will have particles that move randomly. So each gas will spread to fill the container. The movement of the particles of each gas is called DIFFUSION. 3.7 PARTICLES ON THE MOVE Diffusion means the overall random movement of particles from an area where they are in higher concentration to an area where they are in lower concentration. CONCENTRATION is the number of particles in a particular volume. 3.7 PARTICLES ON THE MOVE Tap is closed Blue gas particles High concentration (left) – Zero concentration (right) Red gas particles High concentration (right) – Zero concentration (left) 3.7 PARTICLES ON THE MOVE Tap is opened Gases start to spread out (DIFFUSE) Blue gas particles Quite high concentration (left) – Low concentration (right) Red gas particles Quite high concentration (right) – Low concentration (left) 3.7 PARTICLES ON THE MOVE After some time The gases have completely diffused EQUAL CONCENTRATION of both gases on both sides DIFFUSION STOPS when the concentrations are equal. The movement of individual particles does not stop when diffusion stops. 3.7 PARTICLES ON THE MOVE Example: When food is heated, some particles in the food change state and become gas. The gas particles from the food move randomly and so spread out through the air by DIFFUSION When the concentration of the particles are higher, the smell get stronger as you move closer to the food.  3.7 PARTICLES ON THE MOVE Diffusion also happens in liquids Particles in a liquid also move at random. Example: The blue ink moves from the area of high concentration to the areas of low concentration, until it is completely diffused throughout the water.  3.7 PARTICLES ON THE MOVE The speed of diffusion depends on: 1. The difference in concentration of the particles o The bigger the difference in the concentrations of the particles, the faster the diffusion. 2.Temperature o The higher the temperature, the faster the diffusion o Higher temperature makes particles move faster, so the particles can spread out faster.