Day 51: Faster Than Gravity
We opened with this demo today in AP. There are pink erasers placed on a meterstick every 5 cm. We used the high speed camera to film the drop and watched the video, asked some questions, and discussed what we observed.
“Why do some erasers stay on the meterstick and others don’t?” they asked.
“The erasers that don’t stay on the meterstick all fall together and are lined up horizontally,” they noted.
One student explained why: “The end of the meterstick must accelerating faster than gravity because it falls out from under the erasers. The ‘bend’ in the falling erasers happens at the point where the meterstick is accelerating at 9.8 m/s/s.”
Playing the video back frame-by-frame, we deduced the eraser at 60 cm stays on the meterstick, the one at 65 cm kind of stays on, and the one at 70 cm definately did not stay. Next we determined theoretically where the “bend” should be: 66.7 cm (or 2/3 m).
I have found that by doing demos (concrete) FIRST, followed by the theoretical derivations (abstract), the students see a purpose to the derivation and have some initial footing about the phenomena at hand.
BTW, in College Prep today, we did the colliding carts for visualizing Newton’s Third Law. I already wrote about that here: Newton’s 3rd Law (or How to Make Effective Use of Video for Instruction).