# P4 - Explaining Motion

Revision cards following the OCR 21st Century Science Board. Module P4 - Explaining Motion.

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- Created by: Hallie23
- Created on: 29-03-13 11:41

## Distance-Time Graphs

- Gradient = Speed
- Flat sections are where it's stationary.
- The steeper the gradient, the faster it's going.
- 'Downhill sections mean it's coming back towards its starting point.
- Curves represent acceleration or deceleration.
- A steeping curve means it's accelerating (increasing gradient).
- A levelling-off curve means it's decelerating (decreasing gradient).

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## Displacement-Time Graphs

- When distances are referred to as either positive or negative, it means that an object can be going in one direction or in the opposite direction.
- The displacement of something is its distance in a given direction, from its starting point, at any particular moment in time.

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## Velocity

- The speed of an object is just how fast it's going - the direction isn't important.
- Velocity is sometimes a more useful measure of motion, because it describes both the speed and direction.
- Instantaneous velocity is its speed and direction at a given moment in time.

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## Acceleration and Deceleration

- Acceleration is the change in velocity(or speed) in a certain amount of time.
- Deceleration is negative acceleration.
- Acceleration (m/s squared) = change in velocity (m/s) / Time taken (s).

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## Velocity-Time Graphs

- Gradient = acceleration.
- Flat sections represent moving in a straight line at a constant speed.
- The steeper the gradient, the greater the acceleration or deceleration.
- Uphill sections are acceleration in a straight line.
- Downhill sections are deceleration in a straight line.
- The area under any section of the graph is equal to the displacement travelled in that time interval.
- A curve means changing acceleration.
- Negative velocity means that the object is travelling in the opposite direction.
- Speed-time graphs are similar to velocity-time graphs but ignore direction.

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## Forces

- Force is measured in Newtons.
- A force is a push or a pull.
- Forces come with a force in the opposite direction too.
- When an object exerts a force on another object, it always experiences a force in return. These are called 'partner forces' or 'interaction pairs'.
- An object resting on a surface experiences a reaction force
- Moving object usually experience friction.
- Arrows show the size and direction of forces.
- Resultant forces decide the direction of the motion of the object.

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## Forces and Momentum

- If something accelerates or decelerates, the forces are unbalanced.
- Momentum = Mass x Velocity.
- The heavier the object is, and the faster it's moving, the harder it is to stop.
- The greater the mass of an object, or the greater its velocity, the more momentum the object has.
- Momentum (kg m/s) = Mass (kg) x Velocity (m/s).

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## Change in momentum

- The faster the change in momentum, the greater the chance of injury.
- The change in momentum depends of the force.
- Change in momentum (kg m/s) = Resulatant force (N) x Time for which the force acts (s).
- The bigger the force, the longer it acts for, the bigger the change in momentum.

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## Car Safety

- Force = change in momentum / time
- The grater the time for a chnage in momentum, the smaller the force.
- The smaller the force, the less likely there will be an injury.
- Safety features in a car increase collision time to reduce the forces on the passengers.
- Crumple zones - crumple on impact, increasing the time taken for the car to stop.
- Air bags - slow you down more gradually.
- Seat belts - stretch slightly, increasing the time taken for the wearer to stop. This reduces the forces acting on the chest.
- Cycle and motorcycle helmets - provide padding that inceases the time taken for your head to come to a stop if it hits something hard.

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## Work

- Work done means energy transferred.
- When a force moves an object it does work and energy is transferred to the object.
- Whenever something moves, something else is providing some sort of effort to move it and the 'thing' putting the effort in needs a supply of energy (fuel, food, electricity etc).
- It does work by moving the object and transfers the energy it receives (as fuel) into other forms.
- Whether energy is transferred usefully or is wasted, you cans till say the work is done.
- Amount of energy transferred (J) = Work done (J)
- If energy is transferred, the object doing the work loses energy.
- Work done by a force (J) = Force (N) x Distance moved in direction of force (m)

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## Kinetic Energy

- Kinetic energy is energy of movement.
- The Kinetic energy of something depends on its mass and speed - the greater the mass, the faster it's going, the bigger its kinetic energy.
- Kinetic Energy (J) = 0.5 x mass (kg) x velocity squared ([m/s]squared)
- To increase somethings kinetic energy, you need to increase it's velocity.
- If you do work on an object but it doesn't accelerate, then you havn't increased its kinetic energy.
- Energy is always conserved which means you can't create or destroy energy. Energy just gets transformed from one kind of energy to another.
- The increase in an objects Kinetic energy is normally a bit less than the amount of work done on it because some energy is wasted as heat - unless there is no friction or air resistance acting on an object.

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## Gravitational Potential Energy

- Gravitational potential energy is height energy.
- Gravitational potential energy is the energy stored in an object when you raise it to a height against the force of gravity.
- If you lift and object, its gravitational potential energy increases as it's raised.
- As an object falls, it's gravitational potential energy decreases.
- You increase gravitational potential energy by doing work.
- The increase in gravitational potential energy is equal to the work done by the lifting force in order to raise its height.
- Change in G.P.E. (J) = Weight (N) x Vertical height difference (m)
- When something falls, its gravitational potential energy is converted into kinetic energy.
- Kinetic energy gained is gravitational potential energy lost.

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