Physics & Physical Science Demos, Labs, & Projects for High School Teachers

Inertia Example #517 – The Sledge Hammer & Anvil

Posted on: June 25, 2008

Do I really have 517 demonstrations of inertia? It feels that way, but of course I don’t. I max out at around 300. Anyway, this is similar to putting an anvil on your stomach and then hitting an anvil with a sledge hammer. I’m just not that trusting. Here’s how I made it safe for High School seniors:

I take a box of books. I have a case of Conceptual Physics books, they weigh about 80 lbs. I have a student come up to my demonstration table and lie down. I like to pick one of my more vocal (translate as annoying) but also smaller in frame kids. They lay on their back and I gently place the box of books on their chest and stomach. When they agree they are not in pain, I usually call up the biggest and strongest kid in the class. His job is to slam his fists down on the box of books as hard as he can. This is usually a friend of the one laying down, so the drama is even better.

As hard as he hits the box, the kid on the table barely feels anything. While he’s still there, I ask them about removing some of the books from the box and what do they think would happen. I like for them to take the thought experiment to the extreme, so I ask if we take all the books away and replace it with a single piece of paper. Then what would happen? My hope is that they see how the large mass has a lot of inertia and resists a change in motion.

This usually moves to about half the class trying this on each other at my table. Chaos, you bet, I encourage it. They are interested enough to get out of their chairs and try an experiment, you better believe I’m going to let them. I’ll take every teachable moment I can get.

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