Explain ways of reducing unwanted energy transfer through lubrication

The idea

When two solid surfaces slide over each other, friction does work against the motion and transfers energy by heating to the surroundings, which is usually unwanted. Lubrication reduces this unwanted transfer by placing a layer of oil or grease between the surfaces so that they no longer touch directly. The lubricant lets the surfaces slide more easily, so less work is done against friction and less energy is dissipated by heating. A lower friction force also means less wear on moving parts. Lubrication does not remove friction entirely and does not create energy; it reduces the amount dissipated. In a multiple-choice question, select the option that identifies reduced friction between surfaces as the mechanism, and reject options claiming that lubrication adds energy or stops all energy transfer.

What earns the marks

• Identifies that friction between moving surfaces transfers energy by heating to the surroundings as an unwanted transfer.
• States that a lubricant such as oil or grease forms a layer between surfaces so that they do not touch directly.
• Explains that reducing friction reduces the work done against friction and so reduces the energy dissipated by heating.
• Selects the option that links lubrication to reduced friction and reduced unwanted heating rather than to creating energy or eliminating all transfer.

Common mistakes and how to put them right

• Claims that lubrication creates energy or increases the efficiency beyond the energy supplied.

  Put it right: State that it only reduces the energy dissipated by friction.

• Asserts that lubrication removes friction completely.

  Put it right: Explain that it reduces friction between the surfaces.

• Confuses lubrication with insulation against heat flow.

  Put it right: Describe it as reducing friction between sliding surfaces.

Exam tips

• Read every option and eliminate those that contradict conservation of energy before choosing.
• Link the chosen option explicitly to reduced friction and reduced heating of the surroundings.
• Check that the option names a real mechanism, such as a layer of oil between surfaces, rather than a vague claim.