Work done against the frictional forces acting on an object causes a rise…

The idea

When a force moves an object, energy is transferred mechanically. If friction opposes the motion, some of that transferred energy is dissipated into the thermal energy store of the object and its surroundings, so the object warms up. For example, rubbing your hands together transfers energy from your muscles to the hands' thermal store, making them feel warm. Similarly, a bicycle braking hard converts kinetic energy into thermal energy in the brake blocks and rims. The work done against friction equals the force of friction multiplied by the distance moved along the surface, W = F × d, and this energy appears as a temperature rise rather than as useful kinetic energy.

What earns the marks

• State that work is done when a force moves an object through a distance, using W = F × d.
• Identify friction as a force that opposes motion and acts in the opposite direction to movement.
• Explain that work done against friction is transferred to the thermal energy store of the object and surroundings.
• Link the energy transfer to a measurable temperature rise in the object.
• Use a concrete example, such as brakes heating up or hands warming when rubbed together, to illustrate the process.
• Recognise that the energy is dissipated, meaning it spreads out to the surroundings and becomes less useful.

Common mistakes and how to put them right

• Saying that friction creates energy.

  Put it right by stating that energy is transferred from a kinetic or chemical store to a thermal store, not created.

• Confusing work done with force alone.

  Put it right by emphasising that work done depends on both force and distance moved in the direction of the force.

• Assuming temperature rises only when objects are heated directly.

  Put it right by explaining that mechanical work against friction also raises temperature.

Exam tips

• Always name the energy stores involved, for example kinetic to thermal, rather than saying energy is 'lost'.
• When calculating, check that distance is in metres and force in newtons before multiplying.
• Use the phrase 'work done against friction' explicitly to show the examiner you understand the mechanism.