Concept explainers
* Helge, Steve, and Heidi are sitting on a sled on a slope covered with a hard snow. The sled is stationary. The friends have different suggestions for how to make the sled start moving. Helge If one of us gets off, the sled will start moving:
Steve: We should invite another person to join us, and then the sled will start moving.
Heidi: We should get off the sled, polish the bottom of the sled to make it smoother and sit back down on it. The sled will then start moving.
Comment on the students' suggestions and decide whose reasoning is correct. Explain using physics concepts, including appropriate diagrams, why those who were incorrect said what they did, and indicate what part of their reasoning was correct.
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