# Help with Forces and Pulleys

## Homework Statement

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.

## 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!

Dr. Courtney
Gold Member
2020 Award
Draw a picture. Include free body diagrams.

Express Newton's second law for each object.

Solve for a.

Thewindyfan, Chestermiller and billy_joule
haruspex
Homework Helper
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2020 Award
I used a=(mg-Fparallel)/B,
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?

Thewindyfan, komarxian, Dr. Courtney and 1 other person
Okay, I tried this.

haruspex
Homework Helper
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2020 Award
Mass A is M and mass B is m
That looks fine. You are also asked for the tension.

komarxian
That looks fine. You are also asked for the tension.
So do you know if my answer is correct? Thank you for the help!

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

komarxian
Okay thanks! Will start on the tension now.

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.

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

haruspex
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2020 Award
Im sorry but im pretty sure a is -1.99 ms^-2. i'l put some pics up in a sec
You're right, I didn't notice komarxian had sin and cos mixed up.

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

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