# Homework Help: Tension in the cable of the lift

1. Apr 8, 2012

### jatin1990

1. The problem statement, all variables and given/known data

The tension in the cable supporting a lift moving upward is twice the tension when the lift moves downward. What is the acceleration of the lift?

2. Relevant equations

3. The attempt at a solution
i think its only conceptual but still please show me the way how to think about it.
Thank you.

2. Apr 8, 2012

### tiny-tim

welcome to pf!

hi jatin19901! welcome to pf!

apply good ol' Newton's second law to find the relationship between a g and T for the two cases …

what do you get?

3. Apr 8, 2012

### jatin1990

Re: welcome to pf!

i am doing it like this , when lift is moving downward then , Tension(T) = Mass(m) * Net Acceleration ( Acc. due to gravity(g)+ Lift acceleration(a))
And when lift is moving upward then : 2T = M * (g-a) , and finding out the relation between a and g , i am getting , a=-g/3 , i have a option of g/3 , and i am getting -g/3 .
please correct me where i am wrong.

Last edited: Apr 8, 2012
4. Apr 8, 2012

### tiny-tim

no no no no noooo

never do that, there is only one acceleration for one body

in other words: you can put as many forces as you like on the LHS of F = ma, but you can only put one acceleration on the RHS​

(of course, you can subtract accelerations of different bodies, to get the relative acceleration of the two bodies, but that's not this case)

g is not an acceleration (on the RHS), mg is a force (on the LHS)

try again!

5. Apr 8, 2012

### jatin1990

I do not got your last line.

6. Apr 8, 2012

### tiny-tim

you mean? …
in F = ma, the only a on the RHS is the actual acceleration (what you, for some reason, are calling the "net acceleration" )

on the LHS, you put all the forces, and that includes the weight, mg

7. Apr 8, 2012

### jatin1990

oh ok , yes got the answer as A = g/3 , but i don't know whether it is correct or not , can u please check it. One more thing i have some questions to ask , can you please answer them here or do i need to start a another thread?
Thank you.

8. Apr 8, 2012

### tiny-tim

if you want us to check your calculations, you need to show them
always start another thread

9. Apr 8, 2012

### jatin1990

when lift is moving upward i got , 2T+mg=-ma and when lift is moving downward , T+mg=ma , and then equate the T from both equations , is it right?

10. Apr 8, 2012

### tiny-tim

uhhh?

how can the tension be in the same direction as the weight??​

11. Apr 9, 2012

### jatin1990

Sorry for my stupidity , now i got it. As i am touching physics after two years , so this is the result. I hope i will get my command back upon physics with the help of this forum.
Thank you Tiny Tim for answering my questions very quickly every time.