# P 2.3 Work, energy and momentum

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• Created by: Twins 1&2
• Created on: 12-03-14 19:58

## 3.1 Energy and work

• Whenever an object starts to move, a force must ahve 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 wor done is equal to the energy transferred.
• Both work and energy have unit of joules(J)
• the work done on an object is calculated using the equation
• W = F X d
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## 3.2 Gravitational Potential

Is energy stored in objects because of its position in the Earths 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.

Ep = m x g x h

Power os the rate of transfer of energy.

P = E/t

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## 3.3 Kinetic energy

All moving objects have kinetic energy. The greater the mass and the faster the speed of an object the more kinetic energy it has.

Ek = 1/2 x m x v(2)

An  object  is desctibed as being elastic if it regains its shape after being stretched or squashed. 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 its orginal shape this energy is released

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## 3.4 Momentum

A moving object has momentum. This is the tendency of the object to keep moving in the same direction. It is difficult to change the direction of movement of an object with a lot of momentum.

You can calculate momentum using this equation:

p = m × v

p is the momentum in kilograms metres per second, kg m/s

m is the mass in kilograms, kg

v is the velocity in m/s

For example, what is the momentum of a 5 kg object moving with a velocity of 2 m/s?

Momentum = 5 × 2 = 10 kg m/s

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## 3.7 Car safety

Moving cars have kinetic energy. As energy cannot be created or destroyed, when a car brakes its kinetic energy changes into heat energy. The brakes heat up and then transfer the energy to the surroundings. Modern cars also have safety features that absorb kinetic energy in collisions. These typically include:

seat belts

air bags

crumple zones

All these features reduce injuries to the people in the car by absorbing energy when they change shape.

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