Force spreading out - over area
Teaching Guidance for 14-16
A hockey ball hits a well-padded goalkeeper: less pain results than for a similar strike on an unpadded goalkeeper
Wrong Track: The force is spread out over a larger area, so the head hurts less.
Right Lines: If the ball hits an unprotected body, only a few square centimetres of the surface interact with the ball on impact. Each square centimetre of the ball exerts a large force. When the ball hits the padding, many more square centimetres of the body interact with the ball than in a collision with no padding. More forces now act on the ball, from more square centimetres. Careful engineering of the pads results in each force being smaller.
Perform two separate analyses (with padding, and without). Then compare the findings.
Thinking about the learning
The difficulty here is that there are two separate collisions (one with padding and one without), and they need to be modelled separately, then compared. The temptation is to try to deal with a superposition of both, as a kind of short-cut. That's not a good plan until students are confident using the ideas, and maybe not even then.
Thinking about the teaching
Analyse one collision (without padding). Then analyse the second (with padding). Draw two separate diagrams, one for each analysis. Then compare the analyses. We think that a slower, more thorough comparison is more likely to give a secure understanding of what is being asserted and will set a better pattern for reasoning about new situations.