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How can you tell if there is friction?

Teaching Guidance for 5-11 11-14 Supporting Physics Teaching

Friction and movement

Wrong Track: You only get friction when things move.

Right Lines: Friction exists between all surfaces, moving and tending to move.

Friction with and without movement

Thinking about the learning

This misconception appears in two forms. The first holds that friction happens only when things move (slipping). This idea is based on the conception that friction is about two surfaces rubbing together, and rubbing together is an action requiring motion (slipping). The second is that friction happens only when things don't move (gripping). This is based on the notion that, once moving, an object has overcome friction. Either way the ideas show a limited view of friction.

Thinking about how children go about explaining this difficult idea can help to suggest some approaches.

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Thinking about the teaching

Once again this conceptual challenge can't be resolved by showing friction as a concrete entity to pupils. You can't see something called friction. Through forces spectacles we can draw arrows to show where the friction forces are acting and perhaps this is a good starting place. To show that frictional forces act on stationary objects that are trying to move we can set up many simple situations where a force is acting on a heavy object with the result that it remains in equilibrium. Why isn't it moving even though a force is acting on it? The force which maintains the equilibrium is a grip force, a frictional force.

Some teachers find it helpful to distinguish between slip and grip to make both facets of frictional forces exerted by solids explicit.

Try the same demonstration on an object on wheels or a slippery surface and the result will show that the same external force will indeed have an effect. As with most of these force situations, drawing out pupils' own ideas, often through discussion, is a critical part of any teaching and learning activity.

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