When a force causes an object to move through a distance work is done on…

The idea

Work is the energy transferred when a force makes something move. Two conditions must both hold: a force acts, and the object is displaced along the line of that force. Pushing a wall transfers no energy because the wall does not move, so no work is done. Lifting a book transfers energy to its gravitational store because the upward force moves the book upward. The displacement must have a component in the direction of the force; carrying a bag horizontally while pushing upward does no work against gravity. Work is measured in joules, the same unit as energy, because work is a way of transferring energy. Calculating work done uses force and distance moved along the force direction.

What earns the marks

• State that work is done only when a force causes displacement of an object.
• Identify that both a force and movement in the direction of the force are needed.
• Explain that work done equals energy transferred, measured in joules.
• Recognise situations where a force acts but no work is done because displacement is zero.
• Describe how work done by a force transfers energy between stores.
• Apply the idea to examples such as lifting, pushing or pulling an object.

Common mistakes and how to put them right

• Thinking any force does work.

  Put it right: Displacement in the force direction is also required.

• Believing holding a heavy object still does work.

  Put it right: No displacement means no work done.

• Confusing work with power.

  Put it right: Work is energy transferred, power is the rate of transfer.

Exam tips

• Always check for both a force and movement along the force direction before saying work is done.
• Use the word displacement when explaining why a stationary object has no work done on it.
• Link work to energy transfer and give the unit joule in explanations.