Chapter: 5.3


Work can be done with more or less force. It can be done faster or slower. This is called working with more or less power. When we work with more power, the same work is done faster.


Power is a physical quantity. It is marked with the letter P. It is the ratio of the work done and time taken to do that work.

The unit of power is the watt (W), which means joules per second. A watt is a small unit. In life we often use larger units such as kilowatt (kW), megawatt (MW), gigawatt (GW).

Power is therefore calculated as

Power can be also calculated in another way.

We can calculate power by multiplying the force and speed at which the object is doing the work.

Interesting fact

We can also trace the unit of horsepower, which in Germany and Central Europe was based on the idea that one horsepower (HP) is the amount of power that can lift a load of 75 kg to one meter in one second. It is still used mainly in specifying the power of internal combustion engines in aviation, or in automotive engineering.

Test yourself

Eva stacks food on the kitchen shelf and accomplishes 50 J of work in 10 seconds.

Tim helps her and does 0.3 kJ of work in 1 minute.

Which statement is correct?

b) Eva works with the same power as Tim

The crane takes 18 seconds to lift the load. Calculate the work done by the crane when the load is lifted with a power of 25 kW.

b) 450 kJ

A stationary car with a mass 1.5 t starts moving. After 10 seconds the car’s speed is 30 m/s. What was the average power of the car’s engine?

b) 67.5 kW

Test yourself

Read the statements and determine if they are about work, energy, or power.

Test yourself

Describe the energy transformations when a boy is swinging. Start when he is at the highest point.

Boy has a mass of 30 kg. The height of the swing is 2 m (measured from the ground to the top).

Estimate the difference in height when he is in the lowest and the highest point:  

between 0.8 m and 1.2 m

His gravitational potential energy: 

between 240 J and 360 J

His kinetic energy:  

between 240 J and 360 J

Maximum speed of the boy:

between 4 m/s and 5 m/s

If he wouldn’t move his body during swinging, what would happen?

He would go lower and lower and eventually he would stop swinging.

What energy would its mechanical energy be converted into?

to thermal energy