1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help with Forces and Pulleys

  1. Jan 6, 2016 #1
    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. jcsd
  3. Jan 7, 2016 #2
    Draw a picture. Include free body diagrams.

    Express Newton's second law for each object.

    Solve for a.
     
  4. Jan 7, 2016 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    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.
     
  5. Jan 7, 2016 #4
    Okay, I tried this. image.jpeg
     
  6. Jan 7, 2016 #5
    Mass A is M and mass B is m
     
  7. Jan 7, 2016 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    That looks fine. You are also asked for the tension.
     
  8. Jan 7, 2016 #7
    So do you know if my answer is correct? Thank you for the help!
     
  9. Jan 7, 2016 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes, I agree with your answer.
     
  10. Jan 7, 2016 #9
    Okay thanks! Will start on the tension now.
     
  11. Jan 7, 2016 #10
    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.
     
  12. Jan 7, 2016 #11
    Im sorry but im pretty sure a is -1.99 ms^-2. i'l put some pics up in a sec
     
  13. Jan 7, 2016 #12

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You're right, I didn't notice komarxian had sin and cos mixed up.
     
  14. Jan 7, 2016 #13
    Oh darn, I'll take a look at that. @wrongedMan some pics would help a ton!! Thank you for the help @haruspex as well
     
  15. Jan 7, 2016 #14
    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
     

    Attached Files:

    Last edited: Jan 7, 2016
  16. Jan 7, 2016 #15
    !! Thank you! I misread my formula sheet (again) omo.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted