# Help with Forces and Pulleys

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1. Jan 6, 2016

### komarxian

1. The problem statement, all variables and given/known data
Block A has a mass of 75 kg and is initially at rest on a frictionless inclined plane tilted at a 37 ° above the horizon. Block A is attached to a massless rope that is connected to the top of Block B by way of a massless frictionless pulley. Block B also has a mass of 75 kg and hangs vertically. Find the acceleration of the blocks. Also find the Tension, T, in the rope.

2. Relevant equations

3. The attempt at a solution
When I solved it I found that a = 4 m/s^2, but I think I got it wrong. I used a=(mg-Fparallel)/B, but I may/most likely screwed up here. Any help would be greatly appreciated ;D. I also think that the acceleration might be 0, because it says that it is initially at rest, but that answer also doesn't feel right.
Thank you!

2. Jan 7, 2016

### Dr. Courtney

Draw a picture. Include free body diagrams.

Express Newton's second law for each object.

Solve for a.

3. Jan 7, 2016

### haruspex

You appear to think that adequately explains to others what you did. I assure you it doesn't. E.g. what do you mean by Fparallel? How did you calculate it? What does B represent in the equation - the mass of block B? What about m?

4. Jan 7, 2016

### komarxian

Okay, I tried this.

5. Jan 7, 2016

### komarxian

Mass A is M and mass B is m

6. Jan 7, 2016

### haruspex

That looks fine. You are also asked for the tension.

7. Jan 7, 2016

### komarxian

So do you know if my answer is correct? Thank you for the help!

8. Jan 7, 2016

9. Jan 7, 2016

### komarxian

Okay thanks! Will start on the tension now.

10. Jan 7, 2016

### komarxian

Okay, now that I have the acceleration, I was wondering If what I did to get tension was correct? My teacher recomended using the equation that I used in the second image.

11. Jan 7, 2016

### WrongMan

Im sorry but im pretty sure a is -1.99 ms^-2. i'l put some pics up in a sec

12. Jan 7, 2016

### haruspex

You're right, I didn't notice komarxian had sin and cos mixed up.

13. Jan 7, 2016

### komarxian

Oh darn, I'll take a look at that. @wrongedMan some pics would help a ton!! Thank you for the help @haruspex as well

14. Jan 7, 2016

### WrongMan

well without wanting to give it all away :p ... start with that (img), you should try and solve it without inputing any values just find an expression for a then put values in, repeat for T (bit more tricky for T). I'll still be around for a couple of hours to help you out if you get stuck :)

edit1: BTW, important bit: sin(37) is NOT 4/5 maybe that was your mistake
edit2: yep that was your mistake

edit 3: ah @haruspex i see thats what you meant, i thought you meant he actually wrote cos instead of sin not that he got the values mixed up

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Last edited: Jan 7, 2016
15. Jan 7, 2016

### komarxian

!! Thank you! I misread my formula sheet (again) omo.