NatashaColorado

NatashaColorado
Jul 5, 2009
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I was looking over the questions I got wrong on Physics part of the practice test, and one question surprised me... It seems that I don't understand the meaning of the word "descending" - I thought it means "going down", but appearantly not.
Here is the question:

How much does a 70.0 kg man weigh on scale when standing in a descending elevator that decelerates at 2.50 m/s2? (Assume acceleration of gravity, g = 10.0 m/s2) "


Can anyone, please, explain descending vs ascending? I have no problem with Physics part of it, I just need to understand which way the elevator is going :) My answer was 525 N, but correct answer is 875N (the elevator is going up). In biology, "descending limb of loop of Henle" is the one going down, and every online dictionary confirmed the definition... I wonder if real OAT test has any mistakes?!
 
Last edited:
Jul 24, 2009
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Natasha,
You are correct in that descending means "going down." However, you need to read and think about ALL of the details in the question. It says the the descending elevator DECELERATES! If the elevator which is already in motion in a downward direction begins to decelerate, that means it is slowing down, or actually ACCELERATING in the OPPOSITE (upward) direction! So in essence, you have an elevator accelerating upward against the man, thus leading to the heavier weight.

Make sense?
 

EyEnStein 07

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precisely what live2serve said....

actually decelerating (a physics professor told me) is a poor choice of a word. it should be accelerating because in PHYSICS its just a change in velocity right? dv/dt = "a" ..no matter if the change is + or -

so essentially you have to realize that its saying the elevator is going up...therefore

F= m(g+a) ---> 70(12.5) = 875 N

:thumbup:

i guess its just to trick you. an elevator descending and decelerating is like a double negative isnt it? lol
 
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NatashaColorado

NatashaColorado
Jul 5, 2009
50
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New York, NY
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Optometry Student
Natasha,
You are correct in that descending means "going down." However, you need to read and think about ALL of the details in the question. It says the the descending elevator DECELERATES! If the elevator which is already in motion in a downward direction begins to decelerate, that means it is slowing down, or actually ACCELERATING in the OPPOSITE (upward) direction! So in essence, you have an elevator accelerating upward against the man, thus leading to the heavier weight.

Make sense?
Descending elevator that decelerates has acceleration vector in upward direction - got it! thank you for your help!
 

NeverGiveUp

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The physics section is one of the best things seen in OAT Achiever. You'll get smarter by thinking along the way the author presents the questions and solutions. :thumbup:
 

SonofanOpt

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Jul 31, 2009
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To get the answer you want:
70(10) mass x gravity = 700 (Weight of Man) + 2.5 (700/10) Acceleration UP times the mass.

Like someone above me said, Going down then decelerating by 2.5. This means the force and acceleration are going up and the persons Weight on the scale (which is just the normal force - the upward force), increases.
If the force vector/acceleration was downward - opposite than in this question, and less than gravity (if it was the same as gravity the scale would read zero) it would look like this...

Weight on Scale = 700 - 2.5 (700/10) Which is I believe how you set it up.
 
May 12, 2009
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I just wanted to throw this out there because it was really confusing me for a little bit.

Eq: F=ma
m=70
gravity=10
elevator acceleration= -2.5
a= 10-(-2.5)=12.5

I get what everyone was saying about the situation, but I was getting confused by everyone adding the elevator's acceleration when it was going in the opposite direction.

So I just wanted to clarify for anyone like me who was getting confused... if down is the +y direction then the descending elevator which is deccelerating (accelerating upward) will be going in the -y direction.

That means that when you subtract the opposing accelerations [10-(-2.5)], you actually add them, leading to the scale reading heavier... F= 70*12.5=875.
 

Commando303

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Descending elevator that decelerates has acceleration vector in upward direction - got it! thank you for your help!

But, the "deceleration" is given in positive units; so, shouldn't the elevator be going down...? What a stupidly-worded question.
 

EyEnStein 07

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elevator acceleration= -2.5
a= 10-(-2.5)=12.5

So I just wanted to clarify for anyone like me who was getting confused... if down is the +y direction then the descending elevator which is deccelerating (accelerating upward) will be going in the -y direction.

That means that when you subtract the opposing accelerations [10-(-2.5)], you actually add them, leading to the scale reading heavier... F= 70*12.5=875.
I skipped the step (assumed you guys knew since everyone seemed to be explaining it conceptually..but yes, what you did there is correct. But...its more like something descending is (-) and something decelerating is (-)...because it is slowing down....im not quite sure if decelerating can be the equivalent to accelerating in the opposite direction.
 

SonofanOpt

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Jul 31, 2009
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I skipped the step (assumed you guys knew since everyone seemed to be explaining it conceptually..but yes, what you did there is correct. But...its more like something descending is (-) and something decelerating is (-)...because it is slowing down....im not quite sure if decelerating can be the equivalent to accelerating in the opposite direction.
Yes it is. If you are decelerating you are slowing down (obviously) because the force is going in the opposite direction in order to slow you down. The acceleration vector and the force vector are always aligned.
 

EyEnStein 07

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Yes it is. If you are decelerating you are slowing down (obviously) because the force is going in the opposite direction in order to slow you down. The acceleration vector and the force vector are always aligned.
yeah i suppose...when i wrote that i was thinking more along the lines of a car backing up..but i guess that would be accelerating too in a negative direction depending on axis orientation. The acceleration vector and the velocity vector are always aligned when speeding up and when slowing down acceleration points in the opposite direction of velocity.