# 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

• 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

• 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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