Question

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 get 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 then sit back down 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.

Answer 1

Heidi has the most correct reasoning since if we polish the sled's bottom then the friction between the sled and the surface will decrease and hence the sled will be able to move much more easily. Helge reasoning is also almost on the right track since the force of friction is directly proportional to the normal reaction force on the sled by the ground and by removing the weight the normal reaction force will decrease and hence the friction will also decrease so that the sled can move. Steve is most incorrect out of all since by Inviting another person the weight will increase and so will the friction (unless of course they plan to make the thrid person push them down the slope!) so it will be harder to move the slide.