Day 54: Newton’s 3rd Law and Gravity

Once again, the third law misconception reared its ugly head, despite yesterday’s whiteboard success. I gave students the scenario of a person falling to earth. Like yesterday, I asked for 1 system schema and 2 free body diagrams (one for the falling person and 1 for the earth). Drawing the FBD for the earth was where they had difficulty.

“Does the person pull on the earth?” I asked. Most said no.

We actually hadn’t talked about the nature of gravitational interactions (every mass pulls on every other mass), but I had hoped they might transfer their 3rd Law knowledge from the past few days to this new scenario. Didn’t happen. TRANSFER IS HARD!

So I showed them the above video and we discussed the nature of gravitational interactions.

“So if the forces are equal, then why doesn’t the Earth go flying upward as the person falls downward?” I asked.

Some students were able to make the connection to the Earth having much more mass than the person…just like what happened in the demo when carts of different massess collided and in the demo when a 2 student cart pulled on a 1 student cart. Forces were identical, but the effects of those forces were not.

The conceptual struggles I encounter each day just strengthen my conviction that “great explanations” do little to help students over their hurdles.


About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

4 responses to “Day 54: Newton’s 3rd Law and Gravity”

  1. Trianglemancsd says :

    Sweet video. The lead bricks (like the Earth) don’t move, though. So you’re back in the tug-of-war scenario, right?

  2. Trianglemancsd says :

    By the way, as a physics outsider (I’m a math guy), I’m really intrigued by this thread; I can ask naive questions without having to do very much pretending about my naivete. It’s all real.

  3. Anonymous says :

    Hi Chris,Yep, its the tug-of-war scenario from yesterday. I neglected to describe a bit of the discussion surrounding the video.I asked them to imagine a scenario which ditched the bricks and added a second, identical bar, to the two bars make an X shape. What would happen?"The ends of the bars would move together," they said. (The X would "close up.")"And which bar pulls harder?" I asked."Equally," they said.I asked, "What if I put larger spheres on one of the bars?""The bars would still move together, but the large one wouldn’t move as quickly" they said."And which pulls harder?" I asked."The same, just like the pulling carts from yesterday," they said."So now what about the bricks? Which pulls harder, spheres or bricks?""Equally""So why don’t the bricks move?"Most stumped. I got a few kids saying the earth pulls them down harder than the spheres. (True, but not why the bricks don’t move)"What if the bricks were on wheels?" I asked"They’d move to the spheres," they said."So why do the bricks stay still when there are no wheels?""Friction."*whew*It’s like starting over from scratch each day. (OK, not really, but I’d say if on one day took 4 steps forward, they next day we start 3 steps back. The day-to-day progress is smaller than the bell-to-bell progress.)Keep asking great questions, Chris!

  4. Tim Sheeley says :

    Hello, I teach physics in Iowa and just checking out your N3L stuff, but couldn’t access the gravity video with lead bricks (says not available in your country??)…any suggestions please reply to my email if possible. THX

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