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Finding accleration and tension as an elevator moves upwards/downwards.

  • #1

Homework Statement




A 593 N physics student stands on a bathroom scale in an 875 kg (including the student) elevator that is supported by a cable. As the elevator starts moving, the scale reads 424 N.

A. Find the magnitude of the acceleration of the elevator.

B. What is the acceleration if the scale reads 639 N?

C. What is the tension in the cable in part A?

D. What is the tension in the cable in part B?

Homework Equations



ƩF = ma

T= ma +mg

The Attempt at a Solution



A. 424-593 =60.5a
-2.79 = a

B. 639-593= 60.5a
0.76 = a

C. (875)(2.79)+(593) =T
3034 N = T

D. (875)(.76) + 593
1259 N = T

Is this right? Thanks.

Also, would acceleration stay negative for part C?
 

Answers and Replies

  • #2
256bits
Gold Member
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Lets deal with this question first.
"Also, would acceleration stay negative for part C?"

Well what did you calculate as the acceleration in part a)?
Why would you think that answer would change?

and then parts c) and D)
Which way is the elevator accelerating in part A) and Part B) ie up or down.

If the elevator is accelerating up or down, in which case then would the tension in the cable be greater or lessor than before the elevator started moving?

Draw a FBD to obtain all the forces on the elevator.

And what does this mean in the problem
" 875 kg (including the student) elevator "
 
  • #3
Lets deal with this question first.
"Also, would acceleration stay negative for part C?"

Well what did you calculate as the acceleration in part a)?
Why would you think that answer would change?

and then parts c) and D)
Which way is the elevator accelerating in part A) and Part B) ie up or down.

If the elevator is accelerating up or down, in which case then would the tension in the cable be greater or lessor than before the elevator started moving?

Draw a FBD to obtain all the forces on the elevator.

And what does this mean in the problem
" 875 kg (including the student) elevator "
You're right, part A is negative so I'm guessing I use that same value for part C.

The elevator is accelerating down in part A and up in part B.

Tension would be greater as it accelerates upward.

I will try to re-solve for C and D and answer back, but I think part A and B are right. Thanks.
 
Last edited:
  • #4
ok

I think upwards, T-mg=ma
So, T-8575=665
T= 9240 N

I think downwards, T-mg=-ma
So, T-8575=-2441.25
T= 6134 N

Upwards a=0.760
Downwards a=2.79

Sound better?
 

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