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!

Ropes/Pulleys with moving weights

  1. Feb 22, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the velocity of block B (the one to the right) A has a mass of 3kg and B a mass of 5kg. Pulleys and rope are ideal.

    2. Relevant equations
    F=ma

    3. The attempt at a solution
    I used two different methods, got the same WRONG answer. The correct answer is Vb= -35.8m/s
    Let me know if you can't see what the pictures say

    IMG_2768.jpg
    IMG_2769.jpg
     
  2. jcsd
  3. Feb 22, 2015 #2

    Nathanael

    User Avatar
    Homework Helper

    How is it consistent for T=49.1 N but 2T=29.43 N if it's the same rope? Tension doesn't vary across an ideal rope, because the rope is massless.

    You can't say the tensions are equal to the weights. The objects will accelerate; but if the tension equals the weight, how would the objects accelerate?
    I would leave the tension unknown and eliminate it, no reason to solve for it.
     
  4. Feb 23, 2015 #3
    I used this method:

     
  5. Feb 23, 2015 #4

    Nathanael

    User Avatar
    Homework Helper

    Ah, I didn't see that you wrote "static analysis" above that part.

    In either eq 2 or eq 3, you have an extra factor of -1 on the acceleration term.

    Edit:
    Actually, it could be fixed by slightly altering any of your 3 equations; they just need to be consistent.
    You said, -ab = 2aa which implies that you're letting ab and aa represent the magnitudes and direction (one of them is negative) of acceleration; but then you have an extra factor of -1 on a3 in eq.3, which implies that you're taking aa and ab to be just the magnitude
     
    Last edited: Feb 23, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Ropes/Pulleys with moving weights
  1. Moving Pulleys (Replies: 9)

Loading...