The work done by a force on an object can be calculated using the equation:

The idea

The work done by a force is calculated from the force and the distance moved along the direction of the force. The relationship is work done = force × distance, where force is in newtons, distance in metres and work in joules. A force of 20 N pushing a box 3 m along the floor transfers 60 J. Rearranging gives force = work done ÷ distance or distance = work done ÷ force. The distance must be the displacement along the force direction, not any sideways movement. This equation links the mechanics of forces to energy transfer, so it is used whenever a force moves an object and energy changes store.

What earns the marks

• Recall and use the equation work done = force × distance.
• Substitute force in newtons and distance in metres correctly.
• Calculate work done in joules and give the unit.
• Rearrange the equation to find force or distance when work done is known.
• Use the distance moved along the direction of the force.
• Link calculated work done to energy transferred.

Common mistakes and how to put them right

• Using distance in centimetres without converting to metres.

  Put it right: Convert to metres before calculating.

• Multiplying force by a perpendicular distance.

  Put it right: Use displacement along the force direction.

• Confusing work done with power or using incorrect units.

  Put it right: Work done is a measure of energy transferred and is measured in joules (J), not watts (W).

Exam tips

• Write the equation, substitute values with units, then calculate and state the unit.
• Check whether the question gives distance in cm or km and convert to metres first.
• If asked for force or distance, rearrange before substituting numbers.