# Tag Archives: rotational motion

## More High-Speed Camera Fun

My last post ended with a slow-motion video of falling rolls of paper towels. Here’s a few other videos we’ve taken with the high speed camera:

Ball Bounce Challenge
Students had to predict the drop height necessary in order for the ball to bounce back up to the height of the hoop. Each group was given a different ball. They could take any measurements they wanted with the ball — but the height of the hoop was not disclosed yet. When measurements were complete, the balls were sequestered and the hoop was put in place. Groups then performed more measurements and calculations. Upon determining the drop height, each group was given back their ball and had one chance to make a successful drop:

Reaction Time
Are you quick enough to catch the dollar bill without anticipating?

Falling Meterstick
A classic demonstration. Why do some of the dice stay on the meterstick and some do not? Can you predict how far out along the meterstick the dice will remain in contact with the stick?

Other collections of high-speed video clips

And one more video nicely illustrating Newton’s 1st Law:

Have you been using high-speed videos in the classroom? How?

(NOTE: Some media in this post may not display in feed readers and must be viewed on the website.)

## Falling Rolls

Rotational motion is my favorite topic in AP Physics C: Mechanics. Here’s one reason why:
[taken from Why toast lands jelly-side down: zen and the art of physics demonstrations by Robert Ehrlich]

We did this as a final problem in our study of rotational energy. After working through a series of long equations from energy and kinematics, we discovered the final answer is surprisingly elegant:

UPDATE 12/27/2010: Thanks to Dan Fullerton’s class for catching our error. They analyzed the problem using a torque approach and came up with:

I double-checked our energy approach and now get the same answer as Dan. We must have made an algebra mistake somewhere. This is why I love physics — there is more than one way to solve a problem!

H is the drop height of the free-falling roll, h is the drop height of the unrolling roll, r and R are the inner and outer radii of the unrolling roll.

Using large rolls of paper towels, we tested our prediction. Here’s the result, captured in slow motion:

(Despite our mistake, the demo still works. From the video, it seems the students did not release the rolls simultaneously. Perhaps this compensated for our algebra error.)

They were just a tad excited when it worked. And yes, that class is all boys, much to my dismay.

What’s your favorite activity or demo for rotational motion?