1. Not finding help here? Sign up for a free 30min 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!

Acceleration and Rotational Motion

  1. Oct 11, 2007 #1
    1. The problem statement, all variables and given/known data
    If a rotational object is moving from rest to 200rpm in .23 seconds, the acceleration is?


    2. Relevant equations
    a = V^2/r
    v = 2Pi(r)/t
    I am sure there are others, but this is just a General Physics I course, so its dealing with Newton and Kepler's laws.

    3. The attempt at a solution

    To be honest, I dont know if it is totally solvable. I have a feeling that I am going to be left with a variable in my answer since the radius is not given. Without a radius, I cant figure out velocity, and without velocity I cant figure out acceleration. Judging by previous questions of my professor, the answer would most likely be in meters/second.

    Working with radius as a variable, I am getting something like this:
    a = (2π /.23)² r

    Would this be logical, or am I going about this in the wrong way?
     
    Last edited: Oct 11, 2007
  2. jcsd
  3. Oct 11, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    This problem is asking for angular acceleration, which is measured in radians/s^2. Angular acceleration and velocity are completely analogous to linear acceleration and velocity--the same kinematic equations apply. What's the definition of acceleration? (Be sure to convert from rpm to rad/sec.)
     
  4. Oct 11, 2007 #3
    Ok, so 200 RPM divided by 60 is 3.3 radians/sec (correct?). Since I have the units of radians per second, I should just multiply by 1/t to get an answer in radians/sec², correct?

    3.3 radians/sec * 1/.23s = 14.35 radians/sec²

    Does this sound a little better then?
     
  5. Oct 11, 2007 #4

    Doc Al

    User Avatar

    Staff: Mentor

    No. Rpm stands for revolutions per minute, not radians per minute. How many radians are in one complete revolution?

    But once you get the correct value for rad/s, you have the right idea for finding the average acceleration.
     
  6. Oct 11, 2007 #5
    Ah, if im not mistaken, would it be 2Pi radians in one revolution? So I would multiply 3.3 revolutions/sec by 2Pi to get about 20.73 radians/sec, and then divide by .23 like I did previously and get a final value of 90.15 radians/sec² ?

    Thanks for your help Doc Al.
     
  7. Oct 11, 2007 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Exactly right. (But carry out your intermediate calculations with a bit more accuracy. 3.3 should really be 3.3333... )
     
  8. Oct 11, 2007 #7
    My mistake, I guess I was being a little careless. Thanks again for your help, we definitely did not go over this in lecture. Now that I realize what this is, it is a chapter that we did not cover yet, we were actually going over circular motion instead.
     
    Last edited: Oct 11, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?