Unit 3: Principles of Mechanical Engineering

LO5: Understand principles of dynamic systems

Understanding work, energy and power

Learner activity sheet

Activity 1

In this activity you are going to measure your own power output. To do this you will record your weight, the time it takes you to climb a set of stairs and the total height of the stairs. This will give you enough information to calculate your power output.

The height of the stairs - h metres.

Your body weight - m kg.

The time you take to climb the stairs - t seconds.

You could record two times for this climb:

  • the fastest time you can safely climb the stairs once;
  • the time it takes to walk slowly up the stairs at a pace you could sustain for a long period.

The calculations you need to do are quite simple:

  • Find the increase in your potential energy between the bottom and the top of the stairs:

PE= m  g  h Joules

  • Note that this gives exactly the same result as the work done by your weight being lifted through a vertical distance h.

Therefore: work done on an 'object' is equivalent to the change in energy of that object.

  • Your average power output whilst climbing the stairs is then:

Power = (Increase in energy (or work done))/(time taken) Watts

Notice that the calculation of the increase in PE is independent of time, and the value is the same no matter how quickly or slowly they climb the stairs. It is your power output that represents how quickly the energy is expended.

Activity 2

The activity you have just completed has given an estimate of your power output. This type of study has great relevance to many applications, particularly in fields like sport, and ergonomics. Much more sophisticated methods are used to measure human power output, but how do your figures compare with typical published values?

See or to get started.

Activity 3

In this activity you will find the typical power outputs of typical everyday machines and electrical devices. You may find that power outputs for some machines are often specified in units other than Watts (or kW). For this activity you should convert these units to Watts.

You should find the typical power output (or power consumption of at least the following machines/devices:

  • Motor car (small, family, large).
  • HGV.
  • Railway locomotive.
  • Kettle.
  • Central heating boiler.
  • TV.

How do the power output (or consumption) of these devices compare with your power output when climbing stairs?

Version 1