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Centripetal force question + other question

  1. Dec 11, 2006 #1
    1. A dog sits 1.50 meters from the center of a merry-go-round with an angular speed of 1.20 rad/s. If the magnitude of the force that maintains the dog's circular motion is 40.0 N, what is the dog's mass?

    2. Fc = (mv^2) / r

    3. We were provided with the answer "18.5" but I do NOT understand how you can get this value using the aforementioned equation. I'll demonstrate what I did:
    40 * 1.50 = (1.20^2)m
    60 = 1.44m
    m = 41.6 kg
    can someone explain to me what I'm doing wrong?

    1. An olympic runner leaps over a hurdle. If the runner's initial vertical speed is 2.2 m/s, how much will the runner's center of mass be raised during the jump?

    2. NO IDEA

    3. Don't even know WHERE to begin. Can someone help me out?
  2. jcsd
  3. Dec 11, 2006 #2
    To start off, I'd assume that for the speed, you mean 1.20 m/s, right?

    For your first problem, I'll suggest rewriting the equation. Re-write it to solve for what you're looking for. Then, try it again, and see what you come up with.

    For the second problem, think about what you would need to know to solve the problem, then write them down. This could get you to think of how you could actually solve the problem.
  4. Dec 11, 2006 #3


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    It appears you were given an angular speed of 1.2rad/s. This is NOT the same as linear speed. You must find the equation that relates angular speed to linear speed (v), then plug in that value of v into your correct equation to solve for m.

    In part 2, you need to know either the motion equation that relates velocity, acceleration, and displacement, OR the conservation of energy theorem that relates kinetic and potential energies. Are you familiar with either of these equations?
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