Assemble your students in a room with a smooth floor. Clean asphalt tile
will do nicely. I'll use a basketball court for my example, but someone
may not like you doing this in the gymnasium. I will presume you are
right-footed for kicking, but left-footers can compensate by switching
the instructions because they are brighter.
Place a meter stick on the floor at center court along the midcourt line,
zero to your left and 100 cm to your right. Tell the students you will
give the meter stick a sharp kick perpendicular to the stick at the 5 cm
point. Ask each student to guess where the stick will go when you kick
it, then each student should go to that point and stand there.
When I do this the students guess that the stick will slide off to the
right of the basket somewhere. (I haven't asked them to stand where they
think it will go. I just added that as an afterthought.) Almost all are
surprised when the stick slides right under the basket. Try is again and
this time kick it with a different "oomph". Have them guess again. Fast
or slow, the stick always slides right under the basket.
Why should they be surpised? F = ma is a vector relation after all, and
the force applied to a stick initially at rest would impel it to move
directly toward the basket - in a physics problem, anyway. Should it also
happen in the real world? Not on ice or "assumed frictionless"? Damn
right it should, and after you do it, perhaps they will both know *and*
believe F = ma. (Don't forget those vector signs for your students - I
know that all of you here put them on in your heads.)