Forces and Motion

Something for nothing

Teaching Guidance for 11-14 Supporting Physics Teaching

Getting more out than you put in

Wrong Track: That's brilliant! You get out more than you put in!

Right Lines: It's true that the force to lift the load on the lever is greater than the effort you make, but you must push through a bigger distance to make the load move a small amount. In this way the performance of the lever is limited by the law of conservation of energy.

Talking about trade-offs

Thinking about the learning

Many levers work as force-multipliers. In this case a small force (or effort) acting on the lever moves a large distance to produce a large force moving a smaller distance.

At first this might appear to be a case of 'getting something for nothing'.

Other levers work as distance-multipliers. Here a large force acting on the lever moves a small distance to produce a small force moving a larger distance. This process of compensation for the increase in force or distance is central to the design of levers.

Thinking about the teaching

Here you need to draw attention to the 'trade-offs' between force and distance. We're not dealing with 'something for nothing'. You might make the point more formally by writing it out in the precise way shown below, so that every term is just a number:

energy shiftedjoule = forcenewton × distancemetre

You can also write (making notes to yourself about the units):

energy shifted in joule = force in newton × distance in metre

Or even express it rather concisely as:

energy shifted = force × distance

can be analysed using Torque Moments
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