The big apple
This year I decided to spend some time on explaining how we use graphs to represent relationships in physics before embarking on mechanics. I start by using the example of the relationship between the number of apples and their mass. This brings out the idea of a linear relationship and the physical significance of the gradient. If you put the apples in a box then you get a positive intercept as well. What is more difficult to get across with this example is the idea of uncertainty. If you measure lots of bags of apples you will get a spread in the data but its not due to the uncertainty in the measurement but the fact that apples don't all have the same mass. My students haven't actually been weighing apples but they could have been since we have had a bumper harvest this year.