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: Angular Velocity Calculation

  1. Jun 19, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    v = w x r

    3. The attempt at a solution
    I got the answer correct after a few tries, but I am little confused on why it has to solved this specific way. I know that v = w x r, so why can I not just say w = v/r? Is it because w is measured from the center of the roller so v = w x r would be vcenter, not vtop?
    That is how I ended up solving it, but I wanted to make sure I didn't get there "accidentally" or misinterpreted the process.
    Using that diagram and concept of similar triangles:
    tan θtop = tan θcenter
    Stop/2r = Scenter/r
    Stop = 2*Scenter, Scenter = rθ
    Stop = 2rθ
    d(Stop)/dt = vtop = d(2rθ)/dt = 2rθ' = 2rωtop
    thus, ω = v/2r

    Any reasoning as to why I have to do this would be much appreciated!!
  2. jcsd
  3. Jun 19, 2016 #2
    That's for the centro of the motion is at the bottom of the roller. Your formula should be applied as ##v=2r\cdot\omega,## where ##2r## is the distance from the bottom to the top.
  4. Jun 20, 2016 #3
    Ahh, i forgot, when using v = w x r you have to always measure from from IC unless using relative velocity equation. Thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted