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!

Wheel rotation, constant acceleration

  1. Nov 6, 2009 #1
    1. The problem statement, all variables and given/known data
    A wheel rotates with a constant angular acceleration of 3.71 rad/s2.
    what angle does the wheel rotate between t = 2.00 s and t = 6.25 s?


    2. Relevant equations

    [tex]\omega=\omega_{i}+\frac{1}{2}\alpha t^{2}[/tex]
    Δθ = θf - θi = ωit + αt2
    3. The attempt at a solution
    I found the angular speed at 2 seconds which is 10.01 rad/s.
    Then I use the second equation:
    [tex]\Delta\theta=10.01(6.25)+\frac{1}{2}(3.71)(6.25)^{2}=135.023 rads[/tex]
    I can't get the correct answer. What am I doing wrong?
    Thanks,
    Matthew
     
  2. jcsd
  3. Nov 6, 2009 #2
    The first equation you give is incorrect. It should be [itex] \omega = \omega_i + \alpha t [/itex], so your angular speed at 2 seconds is wrong.

    The second equation should have [itex] \Delta t [/itex] in it instead of t, and also (1/2) inf front of the [itex] \alpha [/itex]
     
  4. Nov 6, 2009 #3
    Oops, yep, I didn't correctly enter the equations. Thanks willem2, I didn't know I needed the change in t for the second equation. I got 76.0484 rads for the angular rotation between t=2s and t=6.25s.
     
  5. Nov 6, 2009 #4

    rl.bhat

    User Avatar
    Homework Helper

    Will you please show your calculations?
     
  6. Nov 6, 2009 #5
    because your answer is unfortunately still wrong.
     
  7. Nov 6, 2009 #6
    Sure, I did this:
    Angular speed at 2s:
    [tex]\omega= \omega_{i}+\alpha t \Rightarrow 2.59 rad/s + (3.71 rad/s2)(2.00 s)=10.01 rad/s[/tex]
    Then the change in t is 4.25s so:
    [tex]\Delta\theta=10.01rad/s(4.25s)+\frac{1}{2}(3.71rad/s^{2})(4.25s)^{ 2}=76.0484 rads[/tex]
    Regards,
    Matthew
     
  8. Nov 6, 2009 #7

    rl.bhat

    User Avatar
    Homework Helper

    From where did you get ωi = 2.59 rad/s?
     
  9. Nov 6, 2009 #8
    Oh, I'm sorry, I forgot that the question stated that at [tex]t=0, \omega_{i}=2.59rad/s[/tex]
     
  10. Nov 6, 2009 #9

    rl.bhat

    User Avatar
    Homework Helper

    Then your answer is correct.
     
  11. Nov 6, 2009 #10
    Thanks for all the help!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook