# GCSE P2 Chapter 3

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• Created by: emma998
• Created on: 07-03-14 16:07

## Energy and work

• Work(Nm) = force(N) X distance(m)
• When a force moves an object energy is transfered and work is done
• If the object is not moved, no work is done
• When work is done moving an object energy is transfered to the object
• Work done to overcome friction is usually transfered to the surroundings by heating.
• When the brakes are applied to a vehicle:
• Friction between the brake pads and the wheel discs oppose the motion of the wheel.
• Kinetic energy of the vehicle is transfered into energy that heats the brake pads and the wheel discs aswell as the surroundings
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## Gravitational potential energy

• Gravitational potential energy is the stored energy an object has due to its position in Earth's gravitational field
• The gravitational field strength of Earth is about 10N
• The higher an object is in Earth's gravitational field strength the greater the gravitational potential energy the object has
• When an object is moved vertically upwards it gains gravitational potential energy equal to the work done on it by the lifting force
• This energy is then released into kinetic energy if the object falls back to the surface of the Earth
• Gravitational potential energy(J) = Mass(Kg) X gravitational field strength(N) X Height(m)
• Power is the rate of transfer of energy
• Power(w) = Energy(J) / time(s)
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## Kinetic energy

• Kinetic energy(J) = 1/2 X mass(kg) X velocity*(m/s)
• Kinetic energy is the energy an object due to its motion
• All moving objects have kinetic energy
• The greater the mass and the greater the velocity of an object the greater the kinetic energy
• Elastic potential enrgy of an elastic object is transfered into kinetic energy when the force deforming its shape is removed
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## Momentum

• Momentum(Kgm/s) = Mass(Kg) X Velocity(m/s)
• All moving objects have momentum
• The greater the mass and the greater the velocity the greater the momentum
• Conservation of momentum: The total momentum before an interaction is the same as after the interaction (change in momentum equals zero)
• Interaction of two objects could be a collision or an explosion
• After an interaction the objects may move off togther or in opposite directions
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## Explosions

• Momentum has both size and direction
• In an explosion one momentum is positive the other is negative
• Because momentum before and after an interaction must be same and before an explosion momentum of objects = zero, momentum of both objects after explosion must be equal and opposite (one + one -)

Firing a bullet from a gun is an example of an explosion:

• Momentum before explosion is zero
• After explosion bullet flies off in one direction
• Gun recoils with equal momentum in opposite direction

Momentum = Mass X Velocity so:

• Bullet has a very low mass therefore velocity is very large
• Gun has same size momentum so because much larger mass, velocity much lower
• Both have same size momentum
• Direction of momentum of bullet is positive, direction of momentum of gun is negative
• Overall momentum after explosion is zero so overall momentum is unchanged
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## Impact forces

• When a force acts on an object that is able to move, the object's momentum changes
• For a paticular change in momentum, the longer the time taken for the change, the smaller the impact force that acts
• In a collision the momentum of an object often becomes zero during the impact - object comes to rest
• Impact forces are increased if the time taken is decreased
• The shorter the impact time the larger the impact force
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## Car safety

• Modern cars contain a number of safety featues designed to reduce impact forces if a collision were to occur
• Side impact bars and crumple zones are designed to fold in slowly, increasing the impact time and reducing the impact forces in a crash
• Seat belts and air bags are also designed to increase impact time by:
• Seat belts stretch slightly increasing impact time reducing impact force on passengers and driver. Also spreads froce equally across the chest
• Air bags also increase impact time and spread force across body. If drivers head hits an airbag momentum changes slowly reducing impact force
• After a crash measurements can be taken from the scene and the law of conservation of momentum applied to work out speeds of the vehicles before collision
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