Force equals motion: motion equals force

Moving and forces

Wrong Track: The ball has a force which keeps it moving through the air. If it's moving it must carry a force.

Right Lines: No constant force is needed to keep an object moving at a steady speed.

Giving forces to things

Thinking about the learning

That moving things need a force to keep them going is a very common misconception. When questioned, a child might typically argue:

Sandra: The trolley is carrying the force you gave it when you pushed it to start it off.

Objects moving with a steady speed are often labelled by pupils with force arrows in the direction of motion. A child might say:

Naz: It will stop when its force is used up.

The idea that a moving object carries a force, usually traced back to the force that was originally applied, is common sense and it is almost correct. A moving object might indeed have been forced to start its motion. However this force was involved in the initial action, the starting push. Once the pushing agent, perhaps a hand or an elastic band, is removed the force is no longer there. The object continues to move but does not carry a force. But the energy in a kinetic store does increase (see the SPT: Energy topic) as a result of the action of the force.

Here are some children talking about moving objects:

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Thinking about the teaching

The big challenge here is to try to eliminate the problems associated with friction. Everyday experience tells us that to keep something moving we need to keep pushing it. This continued application of a force is required simply to overcome the retarding effect of friction. In a world without friction there would be no need to keep pushing. Objects, once in motion, would carry on moving. Pupils don't live in a world without friction. Their experience tells them you need to push to keep things moving.