Physics question: Elevator and weight

[clsmi29]

I saw this question in one of my MCAT practice books:

A person stands on a scale in an elevator. She notices that the scale is reading lower than her normal weight. Which of the following most reasonably describes the motion of the elevator?
A) It is moving down and slowing down.
B) It is moving down at a constant speed.
C) It is moving up and slowing down.
D) It is moving up and speeding up.

I thought the answer was B, but the correct answer is C... Can anyone explain why?! I had this question on a physics exam last semester and my professor said that a scale would read lower than your weight if you were moving down.. So either my professor is wrong, or my practice book. Either way, I'm confused!

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[enervate]

B wouldn't be true because, if she's moving at a constant speed, there is no acceleration. Therefore, the only acceleration present is gravity. As such, the only force present is her normal weight.

Think about A: the elevator is moving down but it is slowing down. Therefore, the acceleration must be acting in the reverse (upward) direction. Of course, you'll recognize that upward acceleration would increase the weight on the scale. Remember that last moment when you're going down on a long elevator ride and it's coming to a stop? You momentarily feel compression or a buckling of your knees. That's what A is describing.

That leaves C (D is obviously wrong). Like the last question, remember that last moment when you're riding an elevator up and it's coming to a stop? You feel weightless for a second. That's the phenomenon the prompt is asking about.

 

[clsmi29]

That makes sense. Great explanation. Thank you!

 

[rueby25]

That makes sense. Great explanation. Thank you!

I will also add that your prof was not necessarily wrong. If the elevator was moving down and speeding up (accelerating), this would also create the described scenario. When all else fails, draw vector diagrams of all forces!

 

[eradicator]

This is a great site that explains all scenarios

http://physics.usask.ca/~kathryn/phys111/apparent_weight.pdf

 

[quechua]

i will also add that your prof was not necessarily wrong. If the elevator was moving down and speeding up (accelerating), this would also create the described scenario. When all else fails, draw vector diagrams of all forces!

+1

 

[petyr_baelish]

I thought about this question as it applies to real life which is always fun and interesting, but I feel that this approach can lead people astray unless you consider all the variables.

For example. what about inertia as an explanation for the feeling of weightlessness when you are moving up in an elevator right as it comes to a stop? Or inertia to explain the feeling of weightlessness when you are moving down in an elevator as it speeds up?

Inertia could be an explanation for the real-world experiences we are wrongly (?) applying to this question. Please correct me if I'm wrong. 😎