Archive for October 2012
I recently picked up a Sony Bloggie camera on eBay for about $50. It’s the same thing as a Flip camera. Make a video, flip out the USB plug, transfer file. It’s just that easy. I set up a backdrop of black paper from floor to ceiling. Kids got on a desk and dropped things. Everybody had to drop a golf ball first. A golf ball will fall pretty close to an ideal parabola, very little air resistance over such a small distance. After that, they could drop anything that wouldn’t break. I have soft squeeze balls and practice whiffle golf balls, superballs and paper balls, and best of all, a coffee filter. The coffee filter is a must, it reaches terminal velocity almost instantly.
We are using Logger Pro software from Vernier. I suppose you could use Tracker, but I have Logger Pro and know how to use it. In Logger Pro, we insert the movie file and then use the tools to place a dot on the object as it drops. The software advances the frame, and in a few minutes we have a synched up video, graph and data table. The software allows the students to quickly see how the slope of the distance vs. time graph changes. They can replay the image and watch their data points in action.
I have them use a quadratic curve fit to calculate the actual acceleration. Then the kids create a second curve and override the fit value with g/2. That puts the expected acceleration curve next to the actual. The effects of air resistance are instantly visible.
We just did this lab for the first time yesterday and today. Give the kids time, it’s going to take them a couple class periods to make this all happen. Initial feedback has been good. I think it’s making sense to them. They can see the effects of acceleration. They can clearly see terminal velocity.
Here is the lab they used. I expect there will be some changes.