P2 Work, Energy and Momentum

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  • Created by: Phoebe
  • Created on: 31-12-12 14:28

3.1 Energy and Work

Work is done on an object when a force makes the object move.

Energy transferred = work done.

W = F x d  

W= work done (j)       F=force (N)                

d=distance moved in direction of force(m)

Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings

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3.2 Gravitational Potential Energy

GPE is energy stored in an object because of its position in the Earth's gravitational field.

The GPE of an object depends on its weight and how far it moves vertically.

The GPE of an object increases when the object goes up and decreases when it goes down.

The change of the GPE of an object (joules)= mass (kg) x gravitational field strength (N/kg) x change of height (metres)

Ep = m x g x h

Power is the rate of transfer of energy

P = E/t   P=power (W)   E= energy (J)   t=time taken (s)

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3.3 Kinetic Energy

The kinetic energy of a moving object depends on its mass and its speed.

Ek = 1/2 x m x v²   Ek = kinetic energy (J)   m=mass (kg)   v=speed (m/s)

Elastic potential energy is the energy stored in an elastic object when work is done on it

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3.4 Momentum

Momentum (kgm/s) = mass x velocity.

Momentum has a size and direction.


Whenever objects interact, the total momentum before the interaction is equal to the total momentum afterwards - provided no external forces act on them.

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3.5 Explosions

Momentum = mass x velocity

When two objects push each other apart, they move apart:

  • with different speeds if they have unequal masses
  • with equal and opposite momentum so their total momentum is zero

Conservation of momentum:

(mass of object A x velocity of A) + (mass of B + velocity of B) = 0

Therefore: mass of A x velocity of A = - (mass of B x velocity of B)

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3.6 Impact Forces

When a force acts on an object that is able to move or moving its momentum changes.

For a particular change in momentum the longer the time taken for the change, the smaller the force that acts.

When two vehicles collide, the force of the impact depends on mass, change of velocity and the duration of the impact.

The longer the impact time is, the more the impact force is reduced.

Crumple zones in cars reduce the forces acting by increasing the time taken to change the momentum of the car

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3.7 Car Safety

Seat belts and air bags spread the force across the chest and also increase the impact time.

Side impact bars and crumple zones fold up in a collision to increase the impact time and reduce the forces acting.

We can use the conservation of momentum law to find the speed of the car before the impact

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