Physics 2A

Physics 2A

Speed, velocity, and graphs
1. Velocity means speed an direction, eg. 30mph north.
2. On a distance-time graph, gradient =speed.
3. On a distance time graph, straight diagonal lines mean a steady speed.
4. On a d-t graph, flat sections are stationary.
5. On a d-t graph, curves represent acceleration or deceleration.
6. Acceleration means how quickly velocity is changing.
7. Acceleration = change in velocity/time taken
8. On a velocity-time graph, gradient + acceleration. The flat sections represent a steady speed. Uphill is accelerating, downhill is decelerating.
9. On a v-t graph, the area under any section is equal to the distance travelled.
10. On a v-t graph, a curve means changing velocity.
Weight, Gravity and Resultant Force
1. Gravity causes all objects to accelerate towards the ground. Earth's gravitational force is 10m/s .
2. Weight is caused by the pull of gravitational force, mass is amount of 'stuff' in an object.
3. Weight = mass X gravitational field strength
4. Resultant force is the overall effect of all the forces acting on an object.
5. If the forces are all equal on the same line, they can balance
6. Result force means a change in velocity, because it changes the state of rest or motion
Forces, Acceleration, and Friction
1. If a stationary object has 0 resultant force, it won't move
2. If an object is moving at a constant speed, the forces on it must be balanced, therefore having 0 resultant force
3. A non 0 resultant force produces acceleration/deceleration. This can be starting, stopping, speeding up, slowing down, or changing direction
4. Resultant force = mass X acceleration
5. When two objects interact, the forces they exert are equal and opposite. However they might move because of a difference in mass.
6. If an object has no force it will stop because of friction, which always acts in the opposite direction.
7. Resistance is called drag in fluids. To avoid this objects are streamlined
8. Friction increases with speed.
9. When an object falls gravity is greater than drag, so it accelerates. Its speed increase causes a build up in friction, which reduces the acceleration.
10. When the frictional force is equal to the accelerating force it can't accelerate anymore. This is terminal velocity
Stopping Distances, Car Design, and Work Done
1. The faster a vehicle is going, the bigger braking force required. The stopping distance will also be larger.
2. Stopping distance = thinking distance + braking distance
3. Thinking distance depends on speed and tiredness/drugs/alcohol, or any other distractions
4. Braking distance is affected by speed, how good your brakes are, how good the tyres are, and how good the grip is (depends on road surface, weather)
5. Work done means energy transferred
6. Work done = Force X Distance
7. Applying the brakes means work is done, it reduces kinetic energy by transferring it into heat
8. Regenerative brakes put the vehicle's motor into reverse, so the wheels are slowed. The motor also acts as an electric generator, changing kinetic energy into electrical energy which is stored as chemical energy
9. Cars have ways of converting kinetic energy safely to prevent a big change in momentum over a short period of time. These are crumple zones, side impact bars, seat belts, and air bags
Potential Energy, Kinetic Energy, and Elasticity
1. Gravitational Potential Energy = mass X gravitational field strength X height
2. Gravitational potential energy is the energy an has because of it height
3. Kinetic energy is movement
4. Kinetic Energy = 1/2 X mass X speed squared
5. Kinetic energy depends on mass and speed, it can be converted to other sorts of energy
6. To stop a car, kinetic energy is converted to heat energy as friction between the wheels and the brake pads. When something falls, its potential energy is converted into kinetic energy, so it speeds up
7. Work done to an elastic object changes its shape, once the force is removed it springs back
8. Extension of an elastic object is directly proportional to force
9. Force = spring constant X extension
10. If the force reaches the limit of proportionality then the object will be permanently stretched; once the force is removed, the material won't spring back
Power, Momentum, and Collisions
1. Power means how much energy is transferred in a space of time, and is measured in watts
2. Power = work done/time taken
3. One watt is one joule transferred per second
4. Momentum is a property of moving objects, the greater mass of an object and the greater its velocity is, the more momentum it will have
5. Momentum = mass X velocity
6. When no external forces act on an event it must have the same amount of energy after as before. This is the Conservation of Momentum
7. When force acts on an object, the momentum changes. A larger force means a faster change in momentum
8. If someone's momentum changes quickly the forces on the body will be large enough to cause injury
9. Fast changes in momentum cause injury so cars are designed with safety features - the longer it takes for a change in momentum to occur, the smaller the force is, therefore making it safer for a person to experience a change in momentum more slowly

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