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Forceful example has mass appeal

The prospect of teaching that "force equals mass times acceleration" with a friction-compensated ramp and an electronic timer, using a walnut trolley with yellow wheels and a set of elastic bands, seemed too optimistic with a class of lively Year 10 girls late on a wintry afternoon. So I made the transition from key stage 3 "push or pull" to "force equals mass..." less problematic.

Using an empty drink can rolling on its side and some volunteer blowers, we easily saw that a force could change the speed or direction of a moving or stationary can.

A swift strike to a rolling can using a hammer convinced the doubters that forces can also change the shape of objects.

They were now more in a mood to be persuaded that force, mass and acceleration might be related.

We blew the can across the bench again, and observed closely its movement, acceleration and terminal speed.

Then I invited two blowers to blow the can - double the acceleration (despite giggling fits when they cottoned on and deliberately blew with less force).

Last, we substituted the empty can with a full one, and they witnessed, with plenty more giggling, the effect of increased mass on the acceleration.

Next lesson, I showed them the trolley system and, working in groups, they eagerly generated sets of results, drew line graphs and produced a model lesson for our school website.

Paul Fullbrook

Chemistry and physics teacher, Dr Challoner's High School, Buckinghamshire

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