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REVISION NOTES: PHYSICS 2
UNIT 3: WORK AND ENERGY
Energy & Work
When a force moves an object, energy is transferred and work is done. When an object starts to
move a force must have been applied to it. This force needs a supply of energy from somewhere,
e.g. from electricity or fuel. When work is done moving the object, the supplied energy is transferred
to the object so the work done is equal to the energy transferred. Both work and energy have the
unit joule, J. The work done on an object is calculated using this equation:
Work done = force x distance moved in the direction of the force
Therefore when the distance moved is nothing, the work done is zero.
An elastic object will go back into its original shape when it has been stretched or squashed. When
work is done on an elastic object to stretch or squash it, the energy transferred is stored as elastic
potential energy. When the object returns to its original shape this energy is released.
Kinetic energy is the energy of movement. The kinetic energy depends on the mass and speed of a
moving object. The greater the mass, and the faster the speed, the more kinetic energy it has. We
calculate kinetic energy using the following equation:
Kinetic energy = ½ (mass x speed²)
Every moving object has momentum. Momentum is measured in kilogrammetres per second, kg
m/s and is calculated using the equation here:
Momentum = mass x velocity
If two objects were to collide, the total momentum before the collision is equal to the total
momentum afterwards (provided no external force acts on them) this is known as the
conservation of momentum. In other words, the total change in momentum before and after
collisions is zero. The same is for explosions. After a collision, the two objects may move off
together in the same direction, or they may separate apart.
As with velocity, momentum has both size and direction. A positive value for the momentum in a
calculation means in the opposite direction to the negative value. Two objects at rest have a
momentum of zero. In an explosion, two objects will move apart with equal and opposite
momentum. One of these momentums will be positive, the other negative and as they share the
same value, the total momentum after the explosion will be zero. An example of an explosion is firing
a gun: as you fire, the bullet moves out with a momentum in one direction, and the gun recoils in the
opposite direction with equal movement.
When a force acts on a moving object (or an object which is able to move), its momentum changes.
The equation below describes this:
Force = change in momentum ÷ time taken for change