# Calculating tension in an elevator

1. Sep 20, 2006

### sweetdaisy186

Hey guys!

I feel like this should be an easy problem....

Two blocks are fastened to the ceiling of an elevator. Each block has a mass of 10 kg. The elevator accelerates upward at 2 m/s^2. Find the tension in each rope.

My prof drew a picture

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2. Sep 20, 2006

### sweetdaisy186

oh whoops, sry I didn't find my question. I thought I could use the T=m*a formula, but that didn't get me the right answer. How do I know which formula to use? Thanks!

3. Sep 21, 2006

### SGT

Have you considered gravity?

4. Sep 21, 2006

### sweetdaisy186

ah! Whoops! I forgot about that! Thanks!

5. Oct 3, 2006

### loudgrrl4_ever

I actually have just about the same problem for an assignment at the moment. The numbers are different, but that doesn't really matter.

I am really stuck.
Since the masses are the same, I am using plain m. Since T=m*a for each rope respectivly, the tension for the top rope should be T=2m*g-2m*a or T=2m(g-a), similarly the tension for the bottom rope would be T=m(g-a).
The problem is, this isn't working.

I am obviously missing something. Any help would be appreciated.

6. Oct 3, 2006

### SGT

Is the elevator going upward or downward?
If it is going upward you should add the accelerations, instead of subtracting.

7. Oct 3, 2006

### loudgrrl4_ever

The elevator is going upward. I don't understand why you would add them, however.
That is the correct answer, but since we have a test tomorrow, I want to be sure I understand it much more than get a point on a silly assignment.

Here's how it is playing out in my mind:
A block is being pulled on upwards directly by the elevator, and downwards by gravity. How does the tension use both of those forces in the same direction?