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Homework Help: Solving angular acceleration Problem

  1. May 21, 2016 #1
    1. The problem statement, all variables and given/known data
    The absolute angle of the thigh has the following angular velocities during the support phase of
    walking. Calculate the angular acceleration at time 0.02s in rad/s and in deg/s

    Time (s) Angular Velocity (rad/s)
    0 s ---- 1.033
    0.02s ------ 1.511
    0.04s ----- 1.882
    0.06s -----2.19

    2. Relevant equations
    i used a= change in v/t but did not get the correct answer

    3. The attempt at a solution
    i subtracted the final velocity from the velocity at 0.02 and did the same with time. so i got 0.679/0.04 but my answer is off. what am i doing wrong ?
  2. jcsd
  3. May 21, 2016 #2
    By doing what you did you get the average acceleration over the time interval t = 0.02 to t = 0.06. You need the angular acceleration at t = 0.02 . I think your best bet is to subtract v at t = 0.04 from the initial velocity to get the average acceleration over the interval t = 0 to t = 0.04 which would be approximately equal to the acceleration at t = 0.02. Does that help?
  4. May 21, 2016 #3
    yes, thank you. by why minus from 0.04s- 0 and not from 0.06 - 0.02 ?
  5. May 21, 2016 #4
    The acceleration is not constant in this case which can be verified by doing a = v/t for different intervals. Therefore in this case a = v/t gives the average acceleration over the time interval t. So for the acceleration at t = 0.02 we need the average acceleration for an interval which include t = 0.02. The best estimate would be the interval t = 0 to t = 0.04. The interval t = 0.02 to t = 0.06 may contain 0.02 but it's at an extreme thus the average over that interval does not represent the acceleration at that point.

    Hope this helps.
  6. May 21, 2016 #5
    thank you. i wanted to ask you when dealing with constant acceleration what equation do you use? do you use

    v(final velocity)= u(initial velocity) + at (acceleration multiplied by time)
  7. May 21, 2016 #6
    Yes. The equation is the same as the one we use except ours will give average acceleration as the acceleration is variable.
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