Elevator Question: Which Situation is True?

[mattr9]

Bob is standing on an ordinary bathroom scale in an elevator. He glances down at the scale, and notes that the reading is greater than his weight. Consider the following situations:

(i) The elevator is traveling upward.
(ii) The elevator is traveling downward.
(iii) The elevator is stationary.

Which situation or situations could be true at the instant Bob glances at the scale? (On Earth.)

 

[||spoon||]

only i) the elevator is moving upwards.

In fact the elevator must be accelerating upwards. If it was to be moving with constant speed upwards the scales would read as normal. If the elecaror was accelerating downwards the scales would read lighter

This is due to the magnitude of the force which the floor exerts upon the person in the elevator and in turn the force which the person exerts on the scales (Newtons third law) when accelerating upwards the force increases and when accelerating downwards the force is less.

||spoon||

 

[mattr9]

nope. I thought it was both 1 and 2 beacuse if the elevator is decelerating downward his apparent weight would be greater also.

BUT the correct answer is all three!

This was a question on my physics test. I personally think it is the trickiest most sneaky question to put on a test. Here is the bull**** explanation my physics teacher gave...

Since the scale reading (normal force) is greater than Bob’s weight, the net force on him must be upwards, and his acceleration must be upwards. He could be traveling upward with increasing speed, downward with decreasing speed, or he could be momentarily stationary as the elevator changes direction from downward to upward. The situation when Bob glances at the scale, then, could be any of situations i, ii, or iii.

 

[blochwave]

That is a SNEAKY question, but it is absolutely right

spoon explained it ABSOLUTELY right and got the important point, it is ONLY the acceleration that matters(unfortunately he didn't quite extend the thought all the way) for that scale reading. In all three cases the elevator could be accelerating in the correct direction to make the scale reading larger

I like it

 

[mattr9]

but how could it be that the apparent weight of him is greater when it is changing directions? at that instant he is stationary (only acceleration is gravity) so he should weigh exactly as he did before.

 

[mgb_phys]

Imagine the elevator falls to the bottom of the shaft, as it hits the bottom it will be stationary. What will the force on his feet be as the elevator hits the ground?

 

[blochwave]

Acceleration does not have to coincide with the value of the velocity at any given point

Just think of circular motion, your linear velocity is 90 degrees different from the centripetal acceleration

 

[||spoon||]

yes your teacher is correct. Sorry I didn't mention the other possible situations, didn't think much into it at the time... eek