Momentous answers to forceful questions
"Force" is one of those words used every day, but which in science has a rather narrow meaning. This dual set of meanings often leads to misconceptions about the causes of movement. In primary school, pupils will learn that forces are "pushes and pulls that make things start or stop moving or change direction", but in secondary school they remain likely to believe that moving objects "have a force travelling with them" - a confusion with the concept of momentum.
Let's help pupils at key stage 3 or 4 sort out what forces actually do. Start by finding out what they already know (the elicitation phase of a lesson) and, more specifically, find out if they have the misconception that moving objects have a "force travelling with them to make them keep moving".
Begin the lesson by asking, and demonstrating, this elicitation question:
"If you throw a tennis ball vertically into the air, it moves upwards from the force from your hand, which has to be greater than gravity (hence the longer upward arrow shown in the diagram, page 7). Neglecting air resistance, mark the force(s) on the ball on its way up."
Give the class 10 seconds to discuss their ideas with a partner, call for silence and ask for responses - "How many others think there is an upwards force?" "Does anyone think there is only a downward force?" "How big is the force of gravity - is your upward force bigger or smaller than gravity?"
Most pupils will want to say there is still a "force from your hand" and may even say that it is bigger than gravity because the ball is still travelling upwards. They are confusing force with momentum. Get the pupils to say what is happening to the speed of the ball - "It is slowing down".
Now produce a tennis ball with a long piece of elastic attached, and throw it horizontally along a bench (but keep hold of the end of the elastic).
The ball slows, stops and returns, just as the ball did when thrown upwards from the hand. In the CD-Rom Science Issues we simply turned the film of a ball being thrown into the air on its side (as in bottom picture). It is now easy to see that there has to be a backwards force pulling on the ball to bring it back. The same thing happens when thrown upwards, but this time it is gravity, rather then the elastic, that slows the ball, and returns it to your hand. Once it has left your hand you have given it momentum, you are certainly not applying a force any more.
We need to probe our pupils' understanding and challenge their deep-seated basic misconceptions.
With a Newtonian view of force, pupils can understand why changing direction or stopping on slippery surfaces is difficult - skidding is caused by a lack of force, so the car or football player just carries on moving. On the CD, this leads on to an understanding of how the atmosphere is held by gravity to the Earth, the first step to understanding its structure and the problems we are causing by throwing away so much material (our waste gases) into it.
l Science Issues and the national curriculum, a resource covering key stage 4 science which addresses misconceptions and sets learning in a real context.
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Keith Ross teaches science education at the University of Gloucestershire