Kinetic energy and work done

A moving object has kinetic energy because work has been done on it.

When work is done energy in one form is transferred to the kinetic energy of the moving object.

To stop the object again, the same amount of work would have to be done to bring it back to rest.

If an object travelling at a certain speed has 2000 J of kinetic energy, we can say that:

2000 J of work has been done in getting the object to travel at that speed from rest

and

2000 J of work would have to be done to bring it back to rest.

curriculum-key-fact
Kinetic energy = work done
Question

A car travelling along a straight road has kinetic energy of 150,000 J.

The breaks are applied, and it is brought to rest over a distance of 65 m.

Calculate the average force of the car breaks.

The car has 150,000 J of kinetic energy.

This means that 150,000 J of work will have to be done on the car to bring it to rest.

i.e. Kinetic energy = work done = 150,000 J

Work done = Fd

Work done = 150,000 J

Distance d = 65 m

150,000 = F x 65 m

F = \frac{150,000 J}{65 m}

F = 2308 N

The average force of the car breaks is 2308 N.