Forces and Motion

Is force the same as energy?

Teaching Guidance for 11-14 Supporting Physics Teaching

Force and energy

Wrong Track: That lorry on the motorway had lots of force, it had lots of energy before it crashed.

Right Lines: A moving object has a kinetic store of energy.

Distinguishing between force and energy

Thinking about the learning

This challenge arises from an imprecise use of specialist terms. A moving object does not have force. However it does have some energy in the kinetic store. The motion will stop when this energy is shifted elsewhere, for example once 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 emptied 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

In helping pupils to untangle this web of language we ought to offer them a clear term which they can use to describe what a moving object has. The default word for many pupils is force. We know that this is incorrect. energy in the kinetic store is totally correct (although momentum might be an acceptable alternative term to use).

At high school an appropriate statement is therefore:

Teacher: A moving trolley has energy in the kinetic store. It will stop when this energy has been shifted from this kinetic store.

The energy shifted to the store as the motion started was through the action of a force (a push).

In their explanations, learners often argue that the force carried by a moving object is the very force which started it in motion. For example:

Abigail: It has all the force which the push gave it.

It is our job to separate the forcing action (the initial push to start the motion) from the energy shifted during the forcing process. Once the force stops acting, the object will be in a new state of motion, having been either slowed down or speeded up by the action of the force.

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