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!

What's wrong with my answers?

  1. Aug 25, 2013 #1
    1. The problem statement, all variables and given/known data

    A block of mass m1 is attached to a block of mass m2 by an ideal rope passing over a pulley of mass M and radius R as shown. The pulley is assumed to be a uniform disc rotating freely about an axis passing through its center of mass (cm in the figure). There is no friction between block 2 and the surface. Assume that the pulley rotates counterclockwise as shown with an angular speed ω and that the rope does not slip relative to the pulley, and that the blocks move accordingly and do not topple or rotate.

    Consider the system to be formed by the pulley, block 1, block 2 and the rope.

    1. Calculate the magnitude of the angular momentum of the system about the center of mass of the pulley. Express your answer in terms of some or all of the variables m1, m2, M, R, ω and g.

    2. Find the pulley's angular acceleration. Express your answer in terms of some or all of the variables m1, m2, M, R, ω and g.

    2. Relevant equations

    angular momentum = moment of inertia x angular velocity
    moment of inertia = mass x radius squared
    torque = moment of inertia x angular acceleration

    3. The attempt at a solution

    for the first part my answer was ((m1+m2)*v*R)+(M*omega*R^2)

    for the second part ((m1*g)/(m1+m2+M))/R

    i just need to know what i'm doing wrong
     

    Attached Files:

    Last edited: Aug 25, 2013
  2. jcsd
  3. Aug 25, 2013 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Shown where?

    This is true for point-masses only, it is not true for disks.

    How do you know it is wrong (it is wrong)? Do you know the solution?
     
  4. Aug 25, 2013 #3
    no i don't know the solution but every time i try to submit my answers i got it wrong.... i only have one more try and cannot figure out what is wrong with my answers???
     
  5. Aug 25, 2013 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    See the comment about point-masses and disks.
    Your moment of inertia of the disk is wrong.
     
  6. Aug 25, 2013 #5
    Also, I hope you realize that, in your angular momentum equation, v = ωR.
     
  7. Aug 25, 2013 #6

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    How are we supposed to tell unless you show your work? Just posting your wrong answers isn't very helpful.
     
  8. Aug 25, 2013 #7

    ehild

    User Avatar
    Homework Helper
    Gold Member

    That is wrong. The moment of inertia of a homogeneous disk is mR2/2.

    ehild
     
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: What's wrong with my answers?
Loading...