Day 28: Do Unequal Masses Pull Equally On Each Other?

College-Prep Physics: Another round of voting, inspired by Preconceptions in Mechanics. Yesterday we determined that the Earth and tennis ball pull mutually on each other. But what about the strengths of the pulls?

VOTE #1 (Target scenario): How do the gravitational forces between the tennis ball & Earth compare? [Almost everyone said F(E on TB) > F(TB on E)]

VOTE #2: (Anchor scenario): How do the gravitational forces between 2 identical tiny masses compare? [Everyone agreed they were equal in size.]

Next we used the whiteboards to draw and label the gravitational forces between tiny mass A and tiny mass X. A surprise to me: most groups were not able to properly label the forces as F(A on X) and F(X on A) — they had the labels reversed. So it was important for me to go to each group and coach them for proper arrow labels and placement (arrows attached to objects, not hanging in mid-air).


How many pulls on A? How many pulls on X?

Then we added attached another tiny mass Y to tiny mass X. Does that change the interaction between A and X? [No.] Do we need to add more forces? [Yes.] Draw them.


How many pulls on A? How many pulls on XY?

Then we added tiny mass Z to mass XY. Do the existing arrows change? Do we have to add more arrows?

I also modeled the situation on the blackboard using magnetic hooks as the masses and rubber bands to represent the gravitational forces between the masses.


VOTE #3 (Bridge scenario): How many forces on A? How many forces on XYZ? So how do the strengths of the gravitational forces compare? [The same?!?]

VOTE #4 (Target scenario revisited): How do the gravitational forces between the tennis ball & Earth compare? [The same?!?]


VOTE #5: Jack weighs 800 N. How hard does Jack pull up on Earth? [800 N]

“But why doesn’t the Earth rise up to meet Jack if the pulls are equal?” Great (anticipated) question from the class. So we talked about how Earth’s 800 N pulling on Jack’s mass has much more of an effect than Jack’s 800 N pulling on the Earth’s entire mass. (And the Earth is like, “Jack, do you even lift?”)

Great lesson in all, though it took a lot of time because I wanted the kids to reach the conclusion on their own rather than me just telling them. I hope it sticks!


NGSS Science and Engineering Practice #6: Constructing Explanations

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About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

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