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).

About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

8 responses to “Day 51: Faster Than Gravity”

  1. MsPoodry says :

    I was totally planning on doing this with my AP class tomorrow, with the high-speed camera too! I hope we get the same result…;-)

  2. Anonymous says :

    Every time you post one of these, I’m more and more convinced that we need to get a high-speed camera. Great explanation to go with the video. I’m really enjoying these short form posts of yours this year.

  3. Josh Ence says :

    What high speed camera are you using? I’d love to buy one for my physics classes!

  4. Anonymous says :

    We use the Casio EX-FH20: video above was shot at 210 fps, which has worked fine for most cases. It takes great photos, too. It even has 2 settings for taking pictures in fluorescent lighting (like most classrooms). There are other models in the Elixim series, too.

  5. Anonymous says :

    Seems like the EX-FH20 is not being made anymore. Looks like Casio just has two high speed models available now:

  6. dethornSTEM says :

    This post drove me nuts. The right end of the meterstick accelerates faster than g because it has a downward acceleration of g and a nonzero leftward acceleration, correct? How did you determine the theoretical bend?

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