Whack a Stack

by Stephanie Chasteen on October 3, 2008

Needing to teach Newton’s Laws?  Don Rathjen, staff educator at the Exploratorium, has been teaching mechanics to students for over 20 years.  This one’s an old favorite — a noisy activity with wood flying everywhere.  You can listen to Don demonstrate how to teach the activity (and geekgirl has some fun with it too) on my Science Teaching Tips podcast.

Here’s a PDF of the activity, and the related Old Tablecloth Trick.

From the activity writeup:

The key concept here is inertia, or resistance to change in motion.  Mass is a measure
of inertia, as shown in Newton’s Second Law, F=ma; for a given force, the larger the
mass, the smaller the acceleration, or change in motion. The “whack” force applied to
the bottom block is far larger than the opposing friction forces from the table and
the remaining block stack, so the bottom cassette undergoes a large acceleration.
Because of the frictional force between the bottom block and block stack
above it, the stack accelerates as well; but the force is small and only occurs for a
very short period of time, and therefore doesn’t give the relatively massive stack much
acceleration before the bottom block is gone. So the stack just drops. Notice as the
blocks are knocked from the stack, the top stack moves farther. Since the stack has
less mass, it has less inertia.

More of Don’s activities here.

{ 1 comment }

David C-L October 3, 2008 at 9:00 pm

That sounded really fun.

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