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.