- #1

ago01

- 46

- 8

I had done this wrong because I had wrongly assumed because the person isn't

*leaving the floor*of the elevator that the net force from N and gravity must balance. After doing the calculations, it was clear N > mg, so there is an acceleration.

Mathematically I can work this out and if I had to regurgitate something on an exam I could do it just from the force diagrams. But I am wondering what the physical manifestation is.

What (I think) I do understand is the concept of the perceived weight of the person. If we stand them on a scale while the elevator is accelerating upwards, they feel heavier. If we do the same as the elevator is slowing down (or going down) they feel lighter. This is also consistent with my experiences. I suppose the problem I'm having is that if there is an imbalance between a surface and the normal force I had this idea that the object would simply fall through that surface (if N < mg) or "bounce" off the surface (if N > mg) and the only reason an object is ever held stationary on a surface is because the normal force exactly balances the gravitational force. This was reinforced by problems that, for example, drop a ball off a cliff. There's no normal force, so the downward acceleration is provided entirely by the gravitational force and the object falls. Or alternatively, a sliding block where it's not leaving the surface because N = mg.

In this elevator though the person isn't being "launched" up as the elevator accelerates. At least this hasn't been my personal experience and I've survived many elevator rides. Also, N > mg. So it seems to me that when moving vertically with a surface the normal force is the force providing the acceleration and no extra forces are required. Then, it would also seem that if I hit a ball with a paddle it's the normal force of the paddle's surface providing the acceleration to the ball at the time of impact (even though we would likely just simply treat this as another force). Is this the right idea?