Day 53: Can You Pull Without Being Pulled In Return? (Updated)

In college-prep we are still exploring Newton’s Third Law. We’ve already done experiments with spring scales and colliding carts. Putting students on rollerblades or the large carts is a nice follow-up.

“Is it possible to pull without being pulled in return?”

“Who pulls harder?”

“What happens if we added another student to one of the carts?”

The misconceptions surrounding the third law are hard to shake, even with the super-visual colliding cart demos we did on Tuesday. Lots of questions came up today, such as, “Then wouldn’t it be impossible to win tug-of-war?” To which we must look at more schema and FBDs to analyze what is going on.

And again, it was too much “me” analysis and not enough them. easy for us when they watch me, but harder for us when I watch them struggle. It just feels so slow…

Any advice? Or just the nature of learning this way for deeper understanding?

UPDATE: I did the following demo with my afternoon class: I had a student holding a force sensor while on a large cart. I was holding a second force sensor while standing on the ground. There was a chain of 2 long rubberbands connecting the two sensors. I pushed the student/cart away from me. He rolled and slowed down as the rubberband stretched and then rolled back to me and accelerated as the rubberband contracted.

I asked, “Are the forces on me balanced or unbalanced? How do you know?” (Balanced because I have constant (zero) velocity.)

I asked, “Are the forces on Ryan balanced or unbalanced? How do you know?” (Unbalanced because Ryan’s velocity changes.)

I asked, “Who pulls harder? Me on Ryan or Ryan on me?” (The same, as seen in the graph of the force sensor readings.)

“So how can we have equal forces that are somehow both balanced and unbalanced?” (Kids were clearly confused.)

Though I desparately wanted to explain it to them, I took a deep breath and asked THEM to get whiteboards. I asked them to draw a schema and 2 FBDs (one for me and one for Ryan). I was worried, but most of them nailed it. Here’s an example:


Tomorrow I’ll try to make the connection of this example to tug-of-war. In this case, Ryan lost the tug-of-war not because he pulled harder than me (he can’t), but because he had no friction.


About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

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