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!

Roational motion help

  1. Dec 13, 2006 #1

    fsm

    User Avatar

    1. The problem statement, all variables and given/known data
    A magnetic computer disk 8.0 cm in diameter is initially at rest. A small dot is painted on the edge of the disk. The disk accelerates at 600 rad/s^2 for .5s, then coasts at a steady angular velocity for another .5s.

    A.What is the speed of the dot at t = 1.0 s? 12m/s
    b. Through how many revolutions has it turned?


    2. Relevant equations
    ummm rotational kinematics...this really isn't the issue


    3. The attempt at a solution

    I was thinking of find theta for the first and second part then adding them, but this did not work. Then i tried to find alpha for the second part 300rad/s^2 then using omega as 300rad/s to get theta at 450


    I am stuck on this problem trying to find the revolutions. I tried the above attempt but it was wrong. I know this is pretty easy but the more and more I try I'm getting frustrated and all knowledge seems to fly out the window.
     
  2. jcsd
  3. Dec 13, 2006 #2

    berkeman

    User Avatar

    Staff: Mentor

    >> ummm rotational kinematics...this really isn't the issue

    Sure it is. Write down the rotational kinematic equations that relate angular position, angular velocity and angular acceleration. Then show your work as you solve for a) and b). If you get stuck, we can offer suggestions by looking at your work.
     
  4. Dec 13, 2006 #3

    radou

    User Avatar
    Homework Helper

    I suggest you write down the equations for uniformly accelerated angular motion first.

    Edit: late again.
     
  5. Dec 13, 2006 #4

    fsm

    User Avatar

    For part a:

    w=w0+at
    w=0+600*.5
    w=300rad/s

    v=w*r
    v=300*0.04
    v=12m/s

    For part b:
    theta=theta0 +w0t+.5at^2
    =0+300+.5*300*1
    =450
    I used the average acceleration. Part b is really a guess.
     
  6. Dec 13, 2006 #5

    berkeman

    User Avatar

    Staff: Mentor

    Try breaking it up into the two parts (accelerating and then coasting) and see if you get the same answers. That would be a good thing to check.
     
  7. Dec 13, 2006 #6

    fsm

    User Avatar

    Isn't it 300 in both parts?
     
  8. Dec 14, 2006 #7

    fsm

    User Avatar

    ok for the accleration:
    theta=theta0+wit+.5at^2
    theta=0+0_.5*600*.5^2
    theta=75rad

    For the coasting:
    theta=.5(wf+wi)t
    theta=.5(300+300).5
    theta=150rad

    total distance=225rad
    revolutions=225/2pi=35.8 revolutions
     
  9. Dec 14, 2006 #8

    radou

    User Avatar
    Homework Helper

    For the coasting you can use the expression [tex]\theta(t) = \omega \cdot t[/tex], where [tex]\omega[/tex] is the angular velocity after the 0.5 sec acceleration.

    Edit: actually, your answer seems to be correct, although I can't understand how you got it, but nevermind.
     
  10. Dec 14, 2006 #9

    BobG

    User Avatar
    Science Advisor
    Homework Helper

    Right answer, but you really only need the first equation. Once for the interval from 0 to 0.5 sec. Once for the interval from .5 to 1 sec. In the second interval, your initial speed is 300 rad/sec for 0.5 sec while your acceleration is 0. It comes to the same answer either way.
     
  11. Dec 14, 2006 #10

    fsm

    User Avatar

    Coolness thanks for the help!!!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Roational motion help
  1. Help with Motion? (Replies: 11)

Loading...