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Homework Help: Help with a statics/dynamics question?

  1. Mar 17, 2015 #1
    Hi all,

    I often find myself on this forum when searching google for help on certain topcis, but I only just joined :)

    I am having trouble with a homework question. The question states:

    If the rod starts from rest in the position shown and a motor drives it for a short time with an angular acceleration of Alpha = (1.5e^t ) rad/s2 , where t is in seconds, determine the magnitude of the angular velocity and the angular displacement of the rod when t = 3s. Locate the point on the rod which has the greatest velocity and acceleration, and compute the magnitudes of the velocity and acceleration of this point when t = 3s. The rod is defined by z = 0.25 sin(Pi y) m, where the argument for the sine is given in radians and y is in meters.

    I have attempted the first section, which I think is correct however it seems too simple/odd to me.

    Here is a link to the actual question (with diagram), and my working so far.

    http://i1374.photobucket.com/albums/ag430/Drvirz/Problem 1_zpsidqrzizz.jpg


    If anyone could confirm this is correct/incorrect and provide an insight into the rest of the question that would be fantastic!

    Thanks in advance!

  2. jcsd
  3. Mar 17, 2015 #2


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    You should have read the Rules of PF when your joined. There is a HW section which is displayed very prominently at the top of the PF website. I'm moving this thread to the Intro Physics HW forum.
  4. Mar 17, 2015 #3
    You appear to have integrated correctly to find the angular velocity function. But it says to solve it for when t=3. As for the next part try to find an equation for the angular displacement of the rod with respect to time.
  5. Mar 17, 2015 #4


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    You forgot the constants of integration in ur equations, should be easy to that the first constant for the angular velocity is -3/2, you should also compute the second for the equation of theta.

    The point with the maximum velocity and acceleration is that with the maximum z. You should find the y for which z becomes maximum. Then the velocity is [itex]ωz_{max}[/itex] and the acceleration is [itex]Alphaz_{max}[/itex].
    Last edited: Mar 17, 2015
  6. Mar 17, 2015 #5
    Never mind you already did that. Then you should ask yourself which part of the rod has the greatest velocity. The equation v= ωr should help...
  7. Mar 17, 2015 #6
    Sorry SteamKing, I will take note for future, and also read the rules.

    Just noticed your second comment. I will try and work through the next section and see how i go
  8. Mar 17, 2015 #7
    Ok so more or less z=0.25sin(Pi*y) for 0<y<1

    So find Z(max) by subbing y-value between 0-1 that will give largest z?
  9. Mar 17, 2015 #8


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    yes but dont forget the constants of integration, the equation for theta becomes a bit different due to the presence of constant of integration in the equation for ω.
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