What happens to movement energy

What happens to movement energy when things speed up or slow down?

When a body speeds up or slows down, its kinetic energy increases or decreases. The forces which cause the change in speed do so by transferring energy to, or from, the body.

Work is done by a force when
a) it makes a body moves or b) it tries to keep it moving or c) it slows it down.


The cars engine provides a thrust force which makes the car move. This force does work. Work done = gain in kinetic energy	When the car is travelling at a steady speed the car's engine provides a force to overcome friction and keep it moving at a steady speed. Work done = work against friction	If the driver takes his / her foot off the pedal then friction will do work to slow the car down. Work done by friction = loss in kinetic energy

When a force causes a body to move through a distance, energy is transferred and work is done.
Work done = energy transferred.

The amount of work done, force and distance are related by the equation:

work done = force applied × distance moved in direction of force or W = F x d
(J) (N) (m)

Work done against frictional forces is mainly transformed into heat.

Elastic Potential Energy

For an object that is able to recover its original shape, elastic potential is the energy stored in the object when work is done on the object to change its shape.

This catapult stores elastic potential energy.

Where did this energy come from?

When the boy lets go what happens to this energy?

Kinetic Energy
The kinetic energy of a body depends on its mass and its speed.


mass = 7 kg speed = 7 m/s K.E. = 172 J	mass = 50g speed = 300 m/s K.E. = 2,250 J	mass = 1,300 kg speed = 30 m/s K.E. = 585,000 J	mass = 30,000 kg speed = 20 m/s K.E. = 6,000,000 J	mass = 50g (of ball) speed = 70 m/s K.E. = 123 J

HT Calculate the kinetic energy of a body using the equation:.

kinetic energy = ½ × mass × speed²
(J) (kg) (m/s)²