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!

Rotational Motion: Disc Movement

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    A 750 gram grinding wheel 25.0 cm in diameter is in the shape of a uniform solid disc. When it is in use it turns at a constant 220 rpm about an axis perpendicular to its face through its center. When the power is switched off the wheel stops in 45.0 s with constant angular acceleration due to friction

    A) What is rotational intertia of the wheel?
    B) What is the initial angular velocity in rad/s?
    C) What is the constant angular acceleration in rad/s^2?
    D) What is the magnitude of the torque exerted by friction in N-m?

    m = 0.750 kg
    r= 0.125 m
    wf = 23.04 rad/s (converting 220rpm)
    t= 45.0 s

    2. Relevant equations
    Part A) Idisc= 0.5(mr^2) = 0.5(0.750kg)(0.125m^2) = 5.86*10^-2 kgm^2


    3. The attempt at a solution

    I was thinking along these lines... I could find part C first (angular acceleration - alpha) and use that to find part B (initial angular velocity - w0) by using this equation : wf = w0 + alpha(time). I could then find part D (torque - T) by using this equation : alpha = T/I

    My problem is finding angular acceleration, or alpha.

    This is what I did, please let me know if it's correct or not.

    v(speed)= radius*wf
    v = (0.125m)(23.04 rad/s) = 2.88
    vf= v0 +a(linear)(time)
    2.88 = 0 +a(45seconds)
    a(linear) = 0.064 m/s
    a(linear) = radius*a(angular)
    0.064 = 0.125*a
    alpha = 0.512 rad/s^2

    If it's wrong, could you please advise on how to get the right answer?
    Thank you in advance :)


    Disclaimers: This is my first post, so please be gentle! Also, this is not homework, it is a practice test. Thanks again!
     
    Last edited: Apr 6, 2009
  2. jcsd
  3. Apr 6, 2009 #2
    Did I read this wrong... I assumed wf= 220rpm = 23.04 rad/s. Is the 220rpm actually w0? And if so, am I just converting it wrong?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Rotational Motion: Disc Movement
  1. Rotating Discs (Replies: 5)

  2. Rotational Movement (Replies: 0)

  3. Rotation of a disc (Replies: 3)

  4. Rotating Thin Disc (Replies: 6)

Loading...