1. Not finding help here? Sign up for a free 30min 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!

Blocks on a Turntable

  1. Sep 20, 2008 #1
    1. The problem statement, all variables and given/known data
    Two identical blocks are tied together with a string and placed along the same radius of a turntable that is spinning about its center. The inner block is 3 cm from the center and the outer block is 5 cm from the center. The coefficient of static friction between the turntable and the blocks is µs = 0.77, and the string is taut.

    a) What is the maximum angular frequency such that neither block slides?
    b) Now suppose that the blocks each have a mass m = 22 g. For the value of w you just found, what is the tension in the string?


    2. Relevant equations
    -T + µmg = m*R1*Theta^2
    T + µmg = m*R2*Theta^2


    3. The attempt at a solution
    To get the answer to part a I used the above 2 equations and solved for theta which gave me w = 13.8 rads/sec. So to solve the for tension in the string I thought I could just plug in numbers for one of the equations and solve for T.. the problem is that each equation gives a different answer for T and they are both wrong. What am I doing wrong?
     
  2. jcsd
  3. Sep 20, 2008 #2
    anyone? I have been working on this last part of this problem for so long now.. ive tried about 10 different answers and they have all been wrong. When I click on help it tells me this:

    Parts (a) and (b) really need to be solved together. Using your free-body diagram, apply F = ma to each block. Remember that the acceleration is the centripetal acceleration (which way is it pointed?). Following this procedure, you will find two equations and two unknowns (the angular frequency and the tension)

    So I don't understand why im able to solve those 2 equations I got for angular frequency but I can't solve them for the tension.. any ideas?
     
  4. Sep 20, 2008 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Redo your calculations for T. If you got different answers from each equation, you made a math error somewhere.
     
  5. Sep 20, 2008 #4
    are the equations right then? when i solved for theta in the equations I did it by adding them together.. to solve for T would I solve the first one for T and then plug that value into the second one for T.. but if I do that then the T's go away which is what is confusing me...
     
  6. Sep 20, 2008 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Yes. (What you call "theta" is the angular speed, usually called "omega".)
    Sure. No problem. That eliminates T and lets you solve for the angular speed.
    You could use either equation to solve for T. You'll get the same answer. (Try it and see.)
    Not sure what you mean here. You might be mixing yourself up a bit. You would plug the angular speed into either of your two equations to solve for T.
     
  7. Sep 20, 2008 #6
    ok.. so the equations are:
    -T + µmg = m*R1*Theta^2
    T + µmg = m*R2*Theta^2

    so:
    -T + (0.77)(22)(9.81) = (22)(0.03)(13.8)^2 so T = 40.491
    T + (0.77)(22)(9.81) = (22)(0.05)(13.8)^2 so T = 43.302

    this is very confusing to me.. you said my equations are right so apparently i must be putting in a wrong number..
     
  8. Sep 20, 2008 #7

    Doc Al

    User Avatar

    Staff: Mentor

    Probably just round-off error. Calculate the value of "theta" to more digits and your two answers will be closer. (Also, the mass is given in grams.)
     
  9. Sep 20, 2008 #8
    im not sure what you mean by the mass is given in grams.. should it be converted to kg?
     
  10. Sep 20, 2008 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Yes, if you want the tension in Newtons.
     
  11. Sep 20, 2008 #10
    ok.. got the answer right now. thanks for all your help! :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Blocks on a Turntable
  1. Blocks on turntable (Replies: 26)

  2. Block on a turntable (Replies: 16)

  3. Blocks on a Turntable (Replies: 4)

  4. Blocks on a Turntable (Replies: 5)

Loading...