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Re: INERTIA DEMO



Tom McCarthy <TMccarthy@steds.org> wrote:
... If anyone has a great inertia demo they wish to share, I would
like to hear about it. ....

AN OLD INERTIA SITUATION AND A DEMO:
Two identical wooden boxes stand on a slippery floor in a moving train.
Box #1 is full of lead while box #2 is full of cotton. Each box has a
seat-belt-like loop around it to prevent forward sliding due to train's
deceleration. But the loops are not strong. The train starts to decelerate,
the loop of the box #1 breakes and the box slides forward (with respect
to the car). The loop of the box #2 does not breake under identical
conditions. A rigid plate of any kind can be used as a "train" in a
classroom demo (+ blocks of different mass + weak sewing threads).
Hitting a wall is an easy way to decelerate. Call it a flying carpet demo,
if a cafeteria try is used.

How can students (who do not know about F=m*a) interpret the result? They
can be guided to conclude that a more massive block has a stronger tendency
to remain in motion than a less massive box. Mass is the inner cause, and
the measure, of the tendency to remain at rest, or to remain in motion.
Newton's laws do form a vicious circle. But we must start somewhere and
proceed linearly. Later students learn that an object which has a mass
happens to attract other mass objects. Yes, the order could be reversed.
But would this facilitate learning?
Ludwik Kowalski