A lot of students will say that if you put a bowling ball on the table, there’s no “normal force” – the table is just “in the way.” So try this first, says Eugenia Etkina.
Have them hold a bowling ball and a tennis ball.
Now draw a picture, using arrows, showing what their hand has to do to hold the tennis ball and the bowling ball.
They’ll automatically draw something that looks like a force diagram, with a larger F for the bowling ball.
Now place the balls on a table and ask them to draw the forces the table has to exert to hold up the balls. They should draw the same thing.
Thus, this physical experience can help them understand that the table does have to exert a force. Plus, the contrasting cases of the bowling ball and tennis ball helps them to discern what is important in these force diagrams. (Note the importance of contrasting cases in Invention Activities, which I’ve written about before – which this activity is similar to).