# Preparation for this topic

In this topic, students can perform some useful experiments with trolleys, weights, pulleys etc. It will help if you familiarise yourself with this equipment in advance, and ensure that the wheels of trolleys and pulleys are as frictionless as possible. Be prepared to cope with experiments where the results don’t quite show that energy is conserved; this will be because of energy losses due to friction. Your students should appreciate that no practical system can be perfect; turn the situation to your advantage by opening up a discussion on the idea of energy efficiency.

## Aims

Students will:

Use the following equations:

• work done = force x distance moved in direction of force.
• potential energy = mgh
• power = work done/ time taken = rate of energy transfer
• power = force x velocity
• efficiency = (useful energy transferred / total work done) x 100%
• Understand energy transfers from potential energy to kinetic energy.
• Equate work done in decelerating to rest with initial kinetic energy.
• Apply these ideas to situations in which vehicles and passengers are accelerated or decelerated.
• Understand and use the principle of conservation of energy (as applied to mechanical energy).

## Prior Knowledge

Students are likely to have a simple understanding of energy, and how it is transferred by a force. They may have met the idea of conservation of energy. Your task as a teacher could then be to extend these ideas to make them more quantitative.

## Rationale

A good grounding in ideas about mechanical energy is needed before going on to a consideration of momentum. General ideas about energy and its conservation permeate the whole of Physics (and, indeed, the whole of science). Many specific results in other areas of science turn out to be examples of energy conservation, e.g. Lenz’s law in electromagnetism, Bernoulli’s equation of fluid flow.

# Work Done by a force

Students have to learn to think of force as a mechanism by which energy is transferred from one body to another. This only occurs when the force moves in the direction of the force.

## Introductory discussions

For some students this will be a revision session. Others may be less familiar with the concepts. Use questions and answers to establish the knowledge of the group. We say that work is done by a force when the object concerned moves in the direction of the force, and the force thereby transfers energy from one object to another. You can be a typical physics teacher and use a board rubber to illustrate your point about energy gain. Alternatively, use some other more interesting object such as a toy airplane.