Force
Forces and Motion | Energy and Thermal Physics

Different measures of motion

Teaching Guidance for 14-16 Supporting Physics Teaching

Force and energy

Wrong Track: That bus had lots of force: it had lots of energy before it hit the pillar.

Wrong Track: Kinetic energy and momentum must be the same: both get bigger as you go faster.

Right Lines: There are two useful measures of the quantity of motion of an object. Neither of these is the force. These measures are the energy in the kinetic store and the momentum.

Distinguishing between force, energy and momentum

Thinking about the learning

This challenge arises from an imprecise use of specialist terms, and perhaps from too much reliance on language alone. There are quantities that are reduced when an object is brought to rest, but none of these is the force, as a moving object does not have force.

Momentum and energy are both reduced when the object is brought to rest. The motion will stop when the energy in the kinetic store is shifted elsewhere – for example, once the brakes have been applied (a thermal store is filled as the brakes and surrounding air are warmed). A free-wheeling vehicle will eventually stop once the frictional forces have shifted all the energy from the kinetic store. In describing such behaviour as running out of force, we can see pupils with almost the right idea but mixing the terms force and energy.

Thinking about the teaching

Elsewhere we've suggested ways of introducing forces that we think will help students identify them accurately (in the SPT: Forces topic). The key point is that they are a way of describing the interactions of the object with the environment. Both momentum and the energy in the kinetic store are properly attributes of the object (at least from a particular point of view), and so could increase or decrease as the object's motion changes. Of course, one is a scalar and one a vector – a distinction probably best made by drawing.

There are fundamental differences between momentum and energy, but we'd suggest that it's far more profitable to focus on seeing how the ideas are used, as it's this experience-based skill that will bring students a good feel for the ideas.

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