Day 75: What Determines Inertia: Mass or Weight?

Today we were comparing the results of the cart and spring lab. Some groups added books to their carts and others didn’t. We noticed that the slope of the acceleration vs. pulling force graph decreased as more books were added.

ME: “Why?”

THEM: “Because Earth pulls down harder on the cart as you add books, making the cart more difficult to accelerate,” they said.

Uh oh.

ME: “So what if we did this on the moon? Would we get different results?”

THEM: “Yes.”

Uh oh.

ME: “You remember the demo where we had 2 carts? One cart had one kid and the other cart had 2 kids? And they pushed off each other? What happened”

THEM: “The cart with 2 kids rolled away more slowly than the cart with one kid.”

ME: “What if we repeated the experiment in space? What would be different?”

THEM: “Since there’s no gravitational pull, they would move apart equally fast.”

Uh oh.

I completely understand why students think like this. It is IMPOSSIBLE to separate mass and weight in everyday experience. Adding mass adds weight.

So I used the demo in the video above. 1st scenario: empty box = velocity changing rapidly = large acceleration. 2nd sceario: box with clamp (more mass and more weight) = velocity changing slowly = small acceleration. Then I asked the kids to make a prediction to the 3rd scenario based on their hypothesis:

IF the acceleration of the box depends on the Earth’s pull on the box,
AND I use a string to hang the clamp from the box (which will reduce the mass of the box but keep the Earth’s pull on the box as when the clamp was in the box),
THEN I would predict the box to have a small acceleration and wobble slowly,
BUT the box wobbles rapidy
SO the acceleration does not depend on the Earth’s pull but rather on the mass of the box.

*whew* — but their conceptions are hard to redirect and I doubt it this demo will enact lasting change in student thinking. But at least I now have a common experience to keep referring back to.

(Coincidentally, Matt Greenwolfe emailed the Modeling list today with the same issue. Glad to see it is universal! 🙂 )

CREDIT goes to physics teaching colleague David Gewanter for showing me this demo several years ago. Thanks, David!


About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

3 responses to “Day 75: What Determines Inertia: Mass or Weight?”

  1. Sue VanHattum says :

    I’ve been skipping most of these physics posts, but this one caught my eye. One of those most basic concepts, that’s hard to get right (for those of us who aren’t physics gurus). I wonder if there’s a thought experiment that would show this to be true (since I’m not going to do the actual hands-on experiment myself).(Bookmarking this. Thanks.)

  2. Anonymous says :

    Hi Sue,I think the thought experiment is where kids were having trouble. Probably the best I could come up with is a situation with an astronaut and a spaceship, since the difference in mass between the two is so extreme. If the astronaut pushes off the space shuttle, do they accelerate at the same rate during the push? I think most kids would say no, perhaps having seen footage of astronauts working in space.Hope that helps. And I hope others chime in with how they address the same situation with students.

  3. Brittany Reed says :

    I will definitely be doing this in my classes. I agree that it probably won’t totally change their thinking, but just spending the time with it and giving them a visual to refer back to should be very helpful. Thanks!

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