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: Circular Motion and magnitude

  1. Jan 27, 2010 #1
    Hi all,

    I'm having a very difficult time with this problem. I know that the magnitude stays the same and it involves uniform circular motion, but I just can't seem to figure out how to begin the problem or what formula to use. I would really appreciate any kind of help.

    A wall clock has a minute hand with a length of 0.47 m and an hour hand with a length of 0.24 m. Take the center of the clock as the origin, and use a Cartesian coordinate system with the positive x axis pointing to 3 o'clock and the positive y axis pointing to 12 o'clock. What is the magnitude of the acceleration of the tip of the minute hand of the clock?

    I also need to express it as a fraction of the magnitude of free-fall acceleration g.

    Again thank you all very much, any kind of help would be so much help.
     
  2. jcsd
  3. Jan 27, 2010 #2

    berkeman

    User Avatar

    Staff: Mentor

    What is the equation for the centripital acceleration of an object (the tip of the clock hand) as a function of radius and velocity?
     
  4. Jan 27, 2010 #3
    Centripetal acceleration's equation is but I've tried plugging the numbers into that equation and can't seem to get it right.

    a= V^2/r
     
  5. Jan 27, 2010 #4

    berkeman

    User Avatar

    Staff: Mentor

    There's another form of that equation in terms of omega. What is the omega of a minute hand on a clock?

    Show us your work...
     
  6. Jan 27, 2010 #5
    I've never done anything with omega unfortunately. But right now I have this:

    a= ((2*pi*.47)/3600)^2/.47
     
  7. Jan 27, 2010 #6

    berkeman

    User Avatar

    Staff: Mentor

    Be sure to show units in your equations -- it helps understandability and helps avoid mistakes.

    Here's a page that should help you understand omega and uniform circular motion better:

    http://en.wikipedia.org/wiki/Uniform_circular_motion

    .
     
  8. Jan 27, 2010 #7
    Thank you very much for the help. I also just do not understand what the second statement, "express it as a fraction of the magnitude of free-fall acceleration g." is looking for.
     
  9. Jan 27, 2010 #8

    berkeman

    User Avatar

    Staff: Mentor

    The magnitude of acceleration due to gravity at the surface of the Earth is generally called "g". Like you feel 1g while standing, and can feel higher g's when stunt flying, etc. Do you know the numerical value of 1g? It's in units of m/s^2.
     
  10. Jan 27, 2010 #9
    Yes, 9.81. So would it be:

    3.86 E-4/9.81 ?
     
  11. Jan 27, 2010 #10
    Although, I do not believe my answer for the first portion of my problem is correct.
     
  12. Jan 27, 2010 #11

    berkeman

    User Avatar

    Staff: Mentor

    Yes, taking the ratio to 9.8m/s^2 would be the way to do the comparison. Post all of your work and I should be able to check it from home in a couple hours (or somebody else can).
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook