# Physics P4 Explaining motion

To help those in need. For the OCR physics 21st

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## How can we describe motion in terms of energy chan

1. Energy is a moving object called kinetic energy

2. As an object falls its gravitational potential energy decreases

3. when a force causes movement work is done

4. Work done by force = force x distance moved in the direction of the force

joule J            newton N                          metre M

5. Change in energy = work done

6. Change in GPE= weight x vertical height difference

joule J              newton N               metre M

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## How can we describe motion in terms of energy chan

7. The heavier and faster an object is moving changes its kinetic energy

8. Kinetic energy = 1/2 x mass x Velocity squared

joules J            kilogram kg       metre per second m/s squared

9. excluding air resistance, a object's kenetic energy will equal the work done to it.

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## What is the connection between forces and motion?

1. When a object is pushed forces are present

2. The resultant force on a object is determined by all of the forces acting on it.

3. Momentum = mass x velocity

kg m/s        kg        m/s

4. change of momentum = resultant force x time for which it has it act

kg m/s                 N                                S

5. Cars and bicycles have a driving force and a counter force (air resistance)

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## What is the connection between forces and motion?

6. If the driving force is larger than the counter force the vehicle will speed up

7. If the counter force is larger the vehicle will slow down

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## What are forces?

1. Forces arise from the interaction between two objects

2. Friction from the road serface affects the forces

3. Rockets and Jet engines use forces to propel them

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## How can we describe motion?

1. Speed = Distance divided by Time taken

m/s            M                             S

2.  Average speed is an average over a short interval

3. Instantaneous speed is Instantaneous

4. When a line is horizontal on a distance/time graph it means that there is no motion taking place

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