Day 27: Is Gravity a Mutual Force?
College-Prep Physics: On Friday, we discussed what causes gravity. Today, we discussed if it was a mutual force. We started by voting on the target scenario: “Does the tennis ball exert a gravitational force on the Earth?” Students shared their thoughts. In one class, everyone said yes, though I doubt everyone actually thought that. So as a way to encourage kids to think about alternate viewpoints, I asked, “Why might a thoughtful person claim that the tennis ball does not exert a gravitational force on the Earth?” In addition, the stigma/fear of sharing an incorrect answer is practically eliminated by framing it as “why might a thoughtful person claim …” rather than “put your hands up if you said … ”
Then we put the target scenario aside and voted on the anchor scenario: “Does Earth 2 exert a gravitational pull on the Earth?” The anchor scenario is one where most kids should have an intuition for the correct answer. We shared our thoughts again and came to consensus. (As an aside, I brought up the notion of a Counter-Earth, and blew a few minds.)
Then we moved to the bridging scenario, where we have the Moon (less massive than Earth 2, but more massive than a tennis ball): “Does the Moon exert a gravitational pull on the Earth?” We shared our thoughts again, several students mentioned tides as evidence, and we came to consensus.
What if we make the mass of the moon smaller and smaller, until it was the same as the mass of a tennis ball? We moved back to the target scenario and re-voted.
Lastly, I asked the students to discuss whether gravity was a one-way or two-way (mutual) force, based on our discussion. I should note that we have not discussed Newton’s Third Law yet. Tomorrow we’ll discuss if the mutual gravitational pulls between unequal masses are equal or unequal in size.
(The sequence of voting questions is based on those found in Preconception in Mechanics.)
NGSS Science and Engineering Practince #6: Constructing Explanations