AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Work done against the frictional forces acting on an object causes a rise…
Full specification wording
Work done and energy transfer: Work done against the frictional forces acting on an object causes a rise in the temperature of the object.
Friction opposes motion, so anything sliding has work done against it, and that work does not vanish - it is transferred to the internal energy store of the two surfaces and they get hotter.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Friction opposes motion, so anything sliding has work done against it, and that work does not vanish - it is transferred to the internal energy store of the two surfaces and they get hotter. Inside a car brake the pads rub the disc, the kinetic store of the car empties, and the internal energy of the brakes rises until they are hot enough to transfer energy to the surrounding air by heating. The same mechanism warms a rope pulled through the hands and a drill bit in metal. At particle level the surfaces are rough, so rubbing makes their particles vibrate faster, and that is what a higher temperature means. Because the energy then spreads into the surroundings it is usually called wasted, although none of it has been destroyed.
What earns the marks
- Stating that work is done against friction between the surfaces.
- The kinetic energy store of the moving object decreasing.
- An increase in the internal, or thermal, energy store, so the temperature rises.
- The energy then being transferred to the surroundings by heating.
Common mistakes and how to put them right
Writing friction alone, which the mark scheme ignores without the work done.
Saying the energy is destroyed or lost rather than transferred to an internal energy store.
Claiming the brakes get hot because of air resistance acting on the car.
Confusing temperature with internal energy when saying what has risen.
Exam tips
- Use the phrase work is done against friction; it is the marking point examiners look for.
- Name internal energy, not heat, as the store that fills.
Try the lesson
The start of the Work done and energy transfer: Work done against the frictional forces acting on an object causes a rise in the temperature of the object. lesson, made from checked teaching notes for this specification point.
Have a go first
A cyclist applies the brakes and the bicycle slows down. What happens to the temperature of the brake blocks?
Listen
Friction opposes motion, so anything sliding has work done against it, and that work does not vanish - it is transferred to the internal energy store of the two surfaces and they get hotter. Inside a car brake the pads rub the disc, the kinetic store of the car empties, and the internal energy of the brakes rises until they are hot enough to transfer energy to the surrounding air by heating. The same mechanism warms a rope pulled through the hands and a drill bit in metal. At particle level the surfaces are rough, so rubbing makes their particles vibrate faster, and that is what a higher temperature means. Because the energy then spreads into the surroundings it is usually called wasted, although none of it has been destroyed. A strong answer earns marks for these points. stating that work is done against friction between the surfaces. the kinetic energy store of the moving object decreasing. an increase in the internal, or thermal, energy store, so the temperature rises. the energy then being transferred to the surroundings by heating. Exam tip: Use the phrase work is done against friction; it is the marking point examiners look for.
See it
Key words
Say what each word means before you turn the card.
work against friction
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internal energy
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temperature rise
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wasted energy
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See every definition
- work against friction
- Work done against the frictional forces acting on an object causes a rise in the temperature of the object.
- internal energy
- The store that fills when work is done against friction, so that the surfaces get hotter.
- temperature rise
- Caused by work done against the frictional forces acting on an object.
- wasted energy
- Energy that spreads into the surroundings, although none of it has been destroyed.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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