Work and power

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What is Work?

When a force is applied to an object, the object moves. Energy is transferred from the source of the force to the object. For example, a person kicking a ball causes the ball to move. The chemical energy used by the person is used to move the leg and the kinetic energy from the leg is transferred to the kinetic energy store of the ball.

The amount of work done to make the object move is equivalent to the amount of energy transferred to the object.

The amount of work done depends upon the size of the force applied to the object and the distance moved by the object. The equation used to calculate work done is:

$\text{work done} = \text{force} \times \text{distance}$

Work done, is measured in joules (J)

Force is measured in Newtons (N)

Distance is measured in metres (m)

You must know and be able to use this equation as it will not be provided in the exam.

Example

A force of 15 N is applied to an object causing it to move a distance of 2 metres.

Calculate the work done in moving the object.

$\text{Work done (joules, J)} = \text{force Newton, N} \times \text{metre, m}$

$\text{Work done } = 15N \times 2m$

$\text{Work done } = 30J$

The energy transferred when the object was moved is also 30 J.

From the equation used to calculate work done, we can see that 1 J of work is done when a force of 1 N is applied through a distance of 1 m in the direction of the force.

Example

Imagine that a car travelling at 40 m/s has an acceleration force of 500 N. The forces of friction opposing the movement are also equal to 500 N.

As the speed of the car is constant, the forces of acceleration and friction must be balanced. The resultant force is equal to 0 N and the car moves in a straight line at a constant speed. To calculate the work done on moving the car forwards in one second, we can substitute the force into our formula for work done:

$\text{Work done (joules, J)} = \text{force Newton, N} \times \text{metre, m}$

$= 500N \times 40m$