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Homework Help: Rotational motion problem, half solved, can't figure out other half.

  1. Mar 21, 2013 #1
    Edit: Nevermind, solved it. 1. The problem statement, all variables and given/known data

    A ceiling fan with 80 cm-diameter blades is turning at 60rpm . Suppose the fan coasts to a stop 25 s after being turned off.

    What is the speed of the tip of the blade 10 seconds after being turned off?

    Through how many revolutions does the fan turn while stopping?

    2. Relevant equations

    It's just kinematic stuff, just rotational, no particularly complex equations.

    3. The attempt at a solution

    Ok, so the first part I could solve just fine.

    I solved for the rotational acceleration using Vf=Vi+at where Vf=0, Vi=pi*.8=2.51 m/s(thanks to the easy conversion of 60rpm to m/s) and t=25s. The acceleration was -.1004. Then I just replugged the values in and solved for Vf at 10 seconds getting 1.5 m/s. That went fine, it was the correct answer.

    The second part is where I am getting trouble. I figured I could just use a general kinematic displacement equation like df=di+vt+.5at^2 to get the distance travelled and then just divide by the circumference....which now that I am typing, I am realizing I put the wrong thing in my calculator, and I just solved it. Lolz. I divided by 2*pi*.8, but .8 is the diameter not the radius. pi*.8 gets me the right answer.

    *sigh,* don't you hate it when you get caught up, not by a lack of understanding, but by just dumb errors? Well thanks for the speedy replies I know I would've gotten. I'll probably be back for more help later.
  2. jcsd
  3. Mar 21, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Well done - at least you didn't just delete the question or leave the actual soution off.
    This way, it is possible someone else will be able to use what you did as an example.
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