Monday, January 12, 2015

Momentum Mobile: Have you seen this equipment?

Demonstration or lab equipment for physics classes can vary from homemade of scraps and junk to store bought and polished shiny pieces. I believe there is a place for each in the classroom but sometimes there is no beating a truly well made purchased piece. An advantage to purchased pieces is that they often stand the test of time and are used year after year (or decade after decade depending on your teaching experience).

Many of us keep a "wish list" of sorts for our classrooms. This may be mental or you may actually have a collection of marked catalogs or ripped out pages. A wise physics teacher (*cough* Dean *cough*) once told me that keeping such a list could help get equipment for your classroom when random money falls from the sky. While it doesn't quite rain I have been lucky enough to have such lists ready at the end of the fiscal year or at the beginning of a new program when an administration is asking for things to buy. That's right, sometimes they ask us and you better be ready!

Yet sometimes you know just what you want to buy but you don't know where to find it. It may have been something you saw at a conference or in a catalog somewhere sometime and you can't quite place it. Whatever it is you just know, "If I had this then my students would completely understand [insert tough concept here]." Perhaps the most frustrating part is that you know the equipment is out there, somewhere, you just don't know where to look. But someone does; maybe avid Blog of Phyz readers?

Another Physics teacher in my district recently said that he wanted this momentum cart he had read about years ago. The cart has a slanted back (higher at the front, lower at the back) and marbles or ball bearings are placed in it. At the start the marbles are stopped from rolling out with a hand at the back of the cart. When you remove your hand the marbles start to roll out and because of conservation of momentum the cart starts to move forward. The marbles fall out one at a time and as they continue to fall the cart speeds up. The teacher said that he thought he remembered the second to the last marble would be the fastest and then the last marble is at rest. This part did not make sense to me but we will have to get the cart to experiment with.

I made this simple image based on his description and he said it looks like what he had seen but didn't know where to find one. Over the years a few students have tried to make it but they have not been successful. Has anyone ever heard or seen anything like this before? Have you seen it for sale anywhere? Perhaps you know what is is properly called and a quick internet search can help us find it. Any and all help would be appreciated!

Bree Barnett Dreyfuss
Amador Valley High School


Ann Hanks said...

You saw this at a Northern California/Nevada AAPT meeting. Scott Perry from American River College made one that launched each ball separately at the same speed. He triggered the release of each ball with a laser pointer.

Dean Baird said...

Well, that's not ringing a bell. But the steel trap of my memory has a few loose teeth, and some things get away.

This one seems to be a straight cascade release.

The motion of balls/cart will depend on the relative masses of each. The lighter the cart and the heavier the balls, the greater the amusement, it would seem.