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!

Homework Help: Need a bit of help -- Pulley accelerated by a force...

  1. Nov 4, 2015 #1
    1. The problem statement, all variables and given/known data
    A pulley, with a rotational inertia of 1.5✕ 10-3kg·m2 about its axle and a radius of 10 cm, is acted on by a force applied tangentially at its rim. The force magnitude varies in time as F = 0.50t + 0.30t2, where F is in newtons and t in seconds. The pulley is initially at rest.

    Find angular velocity at t=7s

    (a) Calculate the torque required to accelerate the Earth in 7 days from rest to its present angular speed about its axis.
    2. Relevant equations

    3. The attempt at a solution

    Okay, for the first one, I integrated the Force and got

    (0.5t^2)/2 + (0.3t^3)/3 = I (alpha) and entered the equation.

    For the second, I'm found the Moment of Inertia of Earth to be 9.74e37 and tried to find angular velocity by this

    (2 (pi))/604800

    The 604800 is the amount of seconds in 7 days on Earth.

    Please help and thank you in advance.
  2. jcsd
  3. Nov 4, 2015 #2
    Don't you need to calculate the current angular velocity of the earth (1 rev) / day?
    Then from that you can calculate the required angular acceleration.
  4. Nov 4, 2015 #3
    @J Hann

    I thought I did with the "2pi/604800" part. So that's velocity? How would I get acceleration?
  5. Nov 4, 2015 #4
    The earth makes 1 (2 pi) revolution in 60 * 60 * 24 = 86400 sec.
    That is the present angular speed.
    Now you need to find the angular acceleration to reach that speed in 7 days!
  6. Nov 4, 2015 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You took an angle, 2pi radians, and divided it by 7 days. That would give you the angular velocity required to rotate once in 7 days.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted