Elevator Motion and Bathroom Scale Readings

[Lucyc2008]

Homework Statement

A physics student . stands on a bathroom scale, calibrated in Newtons, while riding in an elevator. How will the reading on the bathroom scale compare to the normal reading for the following sequence of the elevator's motion? elevator starts to descend; elevator descends at constant velocity; elevator stops descending

Homework Equations

I think this has something to do with Fn (normal force) and velocity.

The Attempt at a Solution

My guess is lighter, normal, heavier as the elevator changes motion because of the velocity/acceleration, for heavier my guess is because the scale or Fn? has to push upward against the person while descending which means more contact against the scale. My guess for lighter is because as the elevator descends the scale drops a little bit.[/B]

 

[gneill]

Hi Lucyc2008,

Welcome to Physics Forums!

My guess is heavier, normal, lighter.

Please show your reasoning for your guesses. What forces are acting?

 

[Cutter Ketch]

My guess is lighter, normal, heavier as the elevator changes motion because of the velocity/acceleration, for heavier my guess is because the scale or Fn? has to push upward against the person while descending which means more contact against the scale. My guess for lighter is because as the elevator descends the scale drops a little bit.

Hmmm ... well your intuition is pretty good, but your explanation is not very good physics. We’ve all been on elevators, so we are all likely to guess correctly based on our experience. I think the trick here is to use what you’ve learned in physics so that you aren’t guessing, but know for sure.

To ground this in some good physics reasoning, don’t think about what the elevator is doing. Think about the motion of the man. Think about what forces are being applied to him, and think about what those forces must be to cause his motion.