Help with Forces and Pulleys

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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.

Homework Equations




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!
 

Answers and Replies

  • #2
Dr. Courtney
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Draw a picture. Include free body diagrams.

Express Newton's second law for each object.

Solve for a.
 
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  • #3
haruspex
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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?
Please post all your steps and define your variables.
 
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  • #4
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Okay, I tried this.
image.jpeg
 
  • #6
haruspex
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Mass A is M and mass B is m
That looks fine. You are also asked for the tension.
 
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  • #7
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That looks fine. You are also asked for the tension.
So do you know if my answer is correct? Thank you for the help!
 
  • #8
haruspex
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So do you know if my answer is correct? Thank you for the help!
Yes, I agree with your answer.
 
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  • #9
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Okay thanks! Will start on the tension now.
 
  • #10
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ImageUploadedByPhysics Forums1452209167.311935.jpg
ImageUploadedByPhysics Forums1452209193.244202.jpg

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
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Im sorry but im pretty sure a is -1.99 ms^-2. i'l put some pics up in a sec
 
  • #12
haruspex
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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.
 
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  • #13
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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
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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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  • #15
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!! Thank you! I misread my formula sheet (again) omo.
 

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