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

Calculate the number of revolutions the disc makes

  1. Dec 8, 2005 #1
    A disc of moment of inertia 29.1 kg m2 is made to rotate about an axis through its centre by a torque of T . The disc starts from rest and, after {t} s, has kinetic energy {k} J.

    The torque is removed when the disc is rotating at 8.0 rad s-1 . An opposing torque of 13.2 N m is applied to slow the disc down.

    Calculate the number of revolutions the disc makes after the second torque is applied, before it comes to rest.

    how do i approch this problem (if numbers were involved) i would have to calculate the angular acceleration and then tention. after t seconds
    any help would be apprecated thanks in advance
  2. jcsd
  3. Dec 8, 2005 #2


    User Avatar
    Homework Helper

    This is a problem in rotational kinematics.

    Could you do it if it were a problem in linear kinematics, in other words, suppose you have a mass of 29.1 kg, with an initial speed of 8 m/s and a resisting force of 32.2 N, could you work out the distance travelled until it stopped ?

    What (kinematic) eqns would you use to solve that problem ?

    The eqns needed for the rotational problem are identical in form with their linear counterparts.

    s = ut + (1/2)at² - linear
    θ = ωt + (1/2)αt² - rotational

    If you know the eqns to use for the linear problem, then just convert them to their rotational equivelents.
  4. Dec 8, 2005 #3
    hi, er... well... i've got the following data...

    a=? (-8/t)

    s= ut + 0.5at^2 ------> s= 4t

    is the initial force equal to the resisting force. so that the force at the end equals 0
  5. Dec 8, 2005 #4


    User Avatar
    Homework Helper

    The resisting force is the decelerating force and is constant throughout the travel, s, of the mass.
    Use newton's 2nd law, F = ma, to get a numerical value for the deceleration. call it a.

    You need to use another kinematic eqn as well, though.
  6. Dec 8, 2005 #5


    User Avatar
    Homework Helper

    It's time I got some sleep (it's 1.00 a.m. here) so it's time I was off. Here's how to do it.

    Use the eqn of motion,

    v² = u² - 2as

    where v is the final velocity (v = 0) and u is the initial velocity (u=8) and a is the deceleration (a = -2.2045 m/s²).

    You will get a distance moved of,

    s = 14.515 m

    Now all you have to do is write out the solution using rotational eqns of motion.

    Substitute for,
    s = θ
    u = ω
    a = α
    The numerical values in your rotational eqns will be identical to those used in the linear eqns and the final angular displacement will be θ = 14.515 rads.
    If you have been having any problems handling rotational eqns of motion, then after you've done it this way a few times, you will be able to just use the rotational eqns of motion straight away.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook