Problems involving someone in an elevator are some of my favorite problems to Whiteboard. It leads to a great discussion of the difference of the force of gravity and how heavy we “feel.” Riding in an elevator and feeling “heavy” or “light” is something that all students can relate to and then connect to the normal force of the elevator floor on the person. Often students have extraneous forces on their free-body diagram (usually the cable supporting the elevator). However, this group drew a great system schema that clearly showed the the cable was not interacting with the person and outside of the system.
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Do you find that students start to change their descriptions of what they feel in an elevator after doing exercises like this? I guess what I mean is, do they at first think that they don’t feel anything, but then after doing this kind of work, they start to notice the jolts.
I don’t think that they change their descriptions, but they better understand them. In fact, initially in the discussions they may be more confused about the forces acting on them in an elevator.
I start the discussion by asking them to describe to someone who has never been in an elevator what it would be like to ride to the top of the Sears Tower. When they describe the elevator accelerating upwards, they use words like “crushing” and “pushed down” which imply a force pushing downwards. They also talk about “feeling heavier.”
At first, they find this confusing as they know there aren’t any forces acting downward on someone in the elevator other than gravity and the force of gravity is constant. However, through the discussion they connect these apparent forces to the normal force exerted upward on the person by the elevator floor.
I hope that this reasoning will pay dividends in the future when we explore “zero gravity” and circular motion.