1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Static Friction problem

  1. Feb 16, 2013 #1
    1. A bug crawls outward from the center of a compact disc spinning at 230 revolutions per minute. The coefficient of static friction between the bug's sticky feet and the disc surface is 1.2. How far does the bug get from the center before slipping?

    2. Relevant equations
    fs ≤ μs n

    3. The attempt at a solution

    I'm trying to get the centrifugal force of the disc. Then substitute the ms by 1.2 and multiply for the force, but I don't remember the formula to get the centrifugal force.
    Last edited: Feb 17, 2013
  2. jcsd
  3. Feb 17, 2013 #2
    The trajectory of the bug (ignoring its slow outward crawl) is a circle. What is acceleration in uniform circular motion?
  4. Feb 17, 2013 #3
    F= m (v^2)/r

    and the velocity is 2pi r w right?
  5. Feb 17, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Usually ω is expressed in radians/sec, which means you don't need the 2pi. But as long as you do the right conversions the above equations are fine.
  6. Feb 17, 2013 #5
    Right, if w is in revolutions per unit of time.
  7. Feb 17, 2013 #6
    r≤ (μs g)/ 4π^2 ω^2 that's where I got stuck right now. g is obviously 9.81 and you said that ω= the 230 revolutions per minute, right?
  8. Feb 17, 2013 #7
    You must use one system of units in the computations. 9.81 uses seconds, 230 uses minutes, which is not compatible. Convert either one.
  9. Feb 17, 2013 #8
    so then I divide 230/60 to get it in seconds?

    now it all makes sense
  10. Feb 17, 2013 #9
  11. Feb 17, 2013 #10
    Thank you very much!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook