1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Rotational Kinematics of Running

  1. Apr 24, 2006 #1
    I need some help.
    The question is:

    At the local swimming hole, a favourite trick is to run horizontally of a cliff that is 8.3m above the water. One diver runs off the edge of the cliff, tucks into a ball and rotates on the way down with an average angular speed of 1.6rev/s. Ignore air resistance and determine the number of revolutions she makes on the way down.

    Now i've done it like this but the anser is wrong.........

    ∆Ө = 1.6rev[(2 pie rad) ÷ (1 revolution)] = 10.05 radians
    Well, gravity pulls her down at 9.8m/s². So in the first second she will fall 9.8m.
    She only needs to fall 8.3m to the water.
    So 8.3m ÷ 9.8m = 0.85s
    It will take her only 0.85s to reach the water.
    And she makes 1.6rev/s.
    So 1.6 ÷ 100 = 0.016
    Then 0.016 x 85 = 1.36
    Thus she will make 1.36 revolutions on her way down.

    What am i missing?
  2. jcsd
  3. Apr 24, 2006 #2
    9.8ms^-2 means that the body will achieve the velocity of 9.81ms^-1 in a second, not 9.81m.
    I think the equation, s=ut+0.5at^2 should be used instead.
  4. Apr 24, 2006 #3


    User Avatar
    Science Advisor

    You are assuming her speed was a constant 9.8 m/s which is not true.
    As Harmony said, use s= ut+ 0.5at^2. Here u (initial speed) is 0 and a (acceleration) is -9.8. Since she is diving down 8.3 m, -8.3= -(0.5)(9.8)t^2.
  5. Apr 24, 2006 #4
    Thanx alot, i dont know how i could have made such a stupid mistake......

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Rotational Kinematics Running Date
The rod OB rotates counterclockwise about O (Polar Coord.) Sep 18, 2017
How to linearise this equation for experiment? May 2, 2017
Pointmass colliding with windmill Apr 18, 2017