Pulleys and Tension

1. Apr 18, 2006

Supernerd2004

Hello,

I have a physics problem most of the way worked out, but i cant seem to get the answer right. I have the problem attached to the post, so hopefully it can provide some insight to my problem.

So far i have:

(Mass A)(gravity) - (Tension 1) = (Mass A)(Acceleration)
(Mass B)(gravity)(sin45)-(Tension 1)+(Tension 2)=(Mass B)(Acceleration)
(Mass C)(gravity)(sin20)-(Tension 2) = (Mass C)(Acceleration)

The problem states that I need to solve for Tension 1, Tension 2 and acceleration.

From there, you I can solve for Tension 1 and 2, then plug them back into the equation for Mass B. I can actually get the right answer, but i have to change the sign on Mass C to get the right answer. If someone could give me a hint to where im going wrong, that would be wonderful. Thanks so much in advance for the help.

Dan

The attachment is still pending approval, so ill try to describe the problem.

There are three boxes with the weight given. They are all attached with massless cables, and connected through a pulley system. The box on the far left is sitting on an incline of 20 degrees. The box weighs 4 pounds. A little farther up, the incline increases to 45 degrees. A pully is placed at the spot where the incline changes. The next block is sitting on this 45 degree incline and it weighs 10 pounds. The incline then peaks, and the last block is suspended in mid-air by a pully. This block weighs 30 pounds. I need to find the tension between the blocks and the acceleration.

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• physics problem.bmp
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Last edited: Apr 18, 2006
2. Apr 18, 2006

Supernerd2004

edit:

I edited the problem, due to the attachment needing approval. Hope this helps,

Thanks again

Dan

3. Apr 18, 2006

nrqed

Hard to tell. But a quick question: if mass A has a positive acceleration (so it accelerates downward, according to your equation), do the other two masses have also a positive acceleration? That does not seem right. Are you sure that you should not either change the sign of the acceleration of A or of both B and C?

4. Apr 18, 2006

Supernerd2004

Thank you so much! The accelerations for b and c needed to be negative. Problem solved!

Thanks again

Dan