# P2.3: Work, energy and momentum

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• Created by: Jordan64
• Created on: 21-04-16 17:45
• P2.3: Work, energy and momentum
• 3.1: Energy and work
• Whenever an object starts to move, a force must have been applied to it.
• When a force moves an object, energy is transferred and work is done
• When work is done moving the object, the supplied energy is transferred to the object so the work done = energy transferred
• Whenever an object is moved vertically upwards it gains gravitational potential energy equal to the work done on it by the lifting force
• Work done to overcome friction is mainly transferred into energy by heating.
• When brakes of a vehicle are applied, friction between the brake pads and the wheel disks opposes the motion of the wheel
• The kinetic energy of the vehicle is transferred into energy that heats the brake pads, the wheel disks and the surrounding air
• 3.2: Gravitational potential energy
• Gravitational potential energy is energy stored in an object because of it's position in the Earth's gravitational field
• Whenever an object is moved vertically upwards it gains gravitational potential energy equal to the work done on it by the lifting force
• Power is the rate of transfer of energy
• 3.3: Kinetic energy
• All moving objects have kinetic energy. The greater the speed of an object, the more kinetic energy it has.
• An object is described as being elastic if it regains it's shape after being stretched or squished.
• When work is done on an elastic object to stretch or squash it, the energy transferred to it is stored as elastic potential energy.
• When the object returns to it's original shape, the energy is released
• When work is done moving the object, the supplied energy is transferred to the object so the work done = energy transferred
• 3.4: Momentum
• All moving objects have momentum.
• The greater the mass and velocity, the greater the momentum
• Whenever two object interact, the total momentum before the interaction is equal to the total momentum afterwards - provided no external forces act on them
• This is called the conservation of momentum
• This means the total change in momentum is zero
• The interaction could be a collision or an explosion. After a collision the objects may move off together, or they may move apart
• 3.5: Explosions
• When two objects are at rest their momentum is zero
• Momentum has both size and direction
• The greater the mass and velocity, the greater the momentum
• In calculations, one direcction must be defined as positive, so momentum in the opposite direction is negative
• In an explosion the objects move apart with equal and opposite momentum.
• One momentum is positive and the other is negative, so the total momentum after the explosion is zero
• Firing a bullet from a gun is an example of an explosion - the bullet moves off with a momentum in one direction and the gun 'recoils' with equal momentum in the opposite direction.
• In an explosion the objects move apart with equal and opposite momentum.
• One momentum is positive and the other is negative, so the total momentum after the explosion is zero
• Firing a bullet from a gun is an example of an explosion - the bullet moves off with a momentum in one direction and the gun 'recoils' with equal momentum in the opposite direction.
• Momentum has both size and direction
• In calculations, one direcction must be defined as positive, so momentum in the opposite direction is negative
• 3.6: Impact forces
• When a force acts on an object that is able to move, or is moving, it's momentum changes.
• For a particular change in momentum the longer the time taken for the change, the smaller the force that acts
• If the impact time is short, the forces on the object are large. As the impact time increases, the forces become less
• When a force acts on an object that is able to move, or is moving, it's momentum changes.
• For a particular change in momentum the longer the time taken for the change, the smaller the force that acts
• If the impact time is short, the forces on the object are large. As the impact time increases, the forces become less
• During the impact of a collision, the momentum of an object comes to rest
• When two objects are at rest their momentum is zero
• Crumple zones in cars are designed to fold in a collision. This increases the impact time and so reduces the force on the car and the people on it.
• Side impact bars and crumple zones fold up in a collision to increase time and reduce the forces acting
• 3.8: Car Safety
• Modern cars contain a number of safety features designed to reduce the forces on the occupants of the car in a collision
• Side impact bars and crumple zones fold up in a collision to increase time and reduce the forces acting
• Seat belts and air bags spread the forces on the body across a larger area.
• If a driver's head hits an airbag it changes momentum slowly, so the force on the head is less than it would be if it changed momentum quickly by hitting the steering wheel
• A seat belt stops the wearer being flung forward if the car stops suddenly.
• The seat belt stretches slightly, increasing the impact time and reducing the force.
• 3.5: Explosions