Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Torque and Net Work

  1. Jul 12, 2010 #1
    1. The problem statement, all variables and given/known data

    An old 33 1/3 rpm turntable may be approximated by a cylindrical disk of mass 0.250kg and a diameter of 31.0cm.

    a.) What average torque will bring it up to operating speed in 1.50 seconds starting from rest?

    b.) what net work is done on the disk in bringing it up to operating speed?

    2. Relevant equations

    3. The attempt at a solution

    Can anyone help me get started on this? We never went over torque!
  2. jcsd
  3. Jul 12, 2010 #2


    User Avatar
    Homework Helper
    Gold Member

    Really? :confused: Is this for a homework problem?

    Well I guess a first step would be to read up on torque. You'll also want to acquaint yourself with the concept of "moment of inertia." As an interim step to this problem, you will need to find the moment of inertia of a 0.250kg cylindrical disk. Before you're finished, you'll also need to research the concept of rotational kinetic energy. :wink:
  4. Jul 13, 2010 #3
    Is this correct?

    mass m = 0.25 Kg
    diameter d = 31 cm
    radius r = 15.5 cm = 0.155 m
    moment of inertia I = (1/2)mr^2 = 0.003 Kg m^2
    ω1 = 0
    n = 33 1/3 = 100/3 = 33.33 rpm = 0.5555 rps
    ω2 = 2πn = 2*3.14*0.5555 = 3.48854 rad/s
    t = 1.5 s
    we know that
    torque τ = I α = I (ω2 - ω1)/t = 0.00697708 N m
    work done w = change in rotational kinetic energy
    = (1/2)*I*(ω2)^2
    = 0.5*0.003*3.48854*3.48854
    = 0.0182 J
  5. Jul 13, 2010 #4


    User Avatar
    Homework Helper
    Gold Member

    'Looks okay to me. :approve:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook