# Physics - P2.3 - Work, Energy and Momentum

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## P2.3.1 - Energy and Work

• Force is applied to anything when it moves - energy is transferred and work is done - object stops moving: supplied energy is transferred to object so work done = energy transferreed
• Work and energy both measured in joules (J)
• W = F x d
• W - work done - J
• F - force - N
• d - distance moved in direction of force - m (no distance moved = no work done on object)
• Work done to overcome friction mainly transferred into energy by heating - brakes applied to vehicle, friction between brake pads and wheel discs oppose wheel motion - kinetic energy of vehicle transferred into energy that heats brake pads, wheel discs and surroundings
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## P2.3.2 - Gravitational Potential Energy

• Gravitational potential energy - energy stored in object because of position in Earth's gravitational field - object moves vertically upwards: gains gravitational potential energy = to work done by lifting force
• Ep = m x g x h
• Ep - change in gravitational potential energy - J
• m - mass - kg
• g - gravitational field strength - N/kg
• h - change in height - m
• Power - rate of transfer of energy
• P = E / t
• P - power - W
• E - energy - J
• t - time - s
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## P2.3.3 - Kinetic Energy

• All moving objects have kinetic energy - greater mass = faster speed of object = more kinetic energy
• Ek = 0.5 x m x v²
• Ek - kinetic energy - J
• m - mass - kg
• v - speed - m/s
• Elastic - regains shape after stretched or squashed - when work is done on elastic object to stretch or squah it, energy transferred is stored as elastic potential energy - when it returns to original shape, energy is released
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## P2.3.3 - Kinetic Energy

• All moving objects have kinetic energy - greater mass = faster speed of object = more kinetic energy
• Ek = 0.5 x m x v²
• Ek - kinetic energy - J
• m - mass - kg
• v - speed - m/s
• Elastic - regains shape after stretched or squashed - when work is done on elastic object to stretch or squash it, energy transferred is stored as elastic potential energy - when it returns to original shape, energy is released
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## P2.3.4 - Momentum

• All moving objects have momentum - greater mass = greater velocity = greater momentum
• p = m x v
• p - momentum - kg m/s
• m - mass - kg
• v - velocity - m/s
• Objects interacting - momentum before interation = momentum after (provided no external forces act on it) - law of conservation of momentum
• Can be described as - total change in momentum = 0
• Interaction could be collision or explosion - after collision objects may move off together or move apart
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## P2.3.5 - Explosions

• Momentum has both size and direction - in calculations, momentum in one direction is positive and negative in the other
• Two objects at rest - momentum = 0, explosions - objects move apart with equal and opposite momentum - one momentum is positive, the other negative - total = 0
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## P2.3.6 - Impact Forces

• Force acts on moving object or object able to move, momentum changes - for particular change in momentum: longer change takes to happen = smaller force acts
• Collision - momentum of object often = 0 during impact: object comes to rest - short impact time = large forces on object - as impact time increases, forces decrease
• Crumple zones in cars - designed to fold in collision - increases impact time: reduces force on car and people in it
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## P2.3.7 - Car Safety

• Modern cars contain safety features designed to reduce forces in occupants of car
• Side impact bars and crumple zones - fold up in collision to increase impact time and reduce forces acting
• Seat belts and air bags - spread forces on body across larger area - if drivers head hits air bag it changes momentum slowly - force on head less than it would be if it changed momentum quickly by hitting steering wheel
• Seat belts - stops wearer being flung forward if car suddenly stops - stretches slightly to increase imapct time and reduce force
• After a car crash, police use measurements from scene and change in momentum to calculate vehicles speed before collision
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