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Homework Help: Help understanding circular motion

  1. Aug 2, 2010 #1
    1. The problem statement, all variables and given/known data
    A skateboarder with mass 61kg sliding down a circular ramp reaches a speed of 7.8 m/s at the bottom of the ramp
    what is the net force acting on the skateboarder at that point?

    2. Relevant equations

    N = mg
    Fr = m * v^2/r

    3. The attempt at a solution
    In my opinion the only force acting on the skateboarder at the bottom of the ramp is the NORMAL force which is m*g = 597N

    but since the answer in the book is 1127 it seems it takes into account some fictional centrepintal force which is m * v^2/r = 530

    530+597 = 1127

    however as far as I can tell there is no actual force pulling him inward towards the center of the ramp but and the only reason he stays in a "circular motion" at that point is the solid ramp preventing his escape, now if we were at the top of the ramp it would have been a different matter.

    I also have a second question deriving from this one, if I were asked what are the net forces acting on this skateboarder not at the bottom or the top, but rather at an angle, say 30 degrees from center where 0 is the bottom.

    infact if someone could explain all kinds of degrees,say... 30, 90 and 130? how do I even aproach that?
  2. jcsd
  3. Aug 2, 2010 #2

    Doc Al

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    Staff: Mentor

    m*g is the weight of the skateboarder, not the normal force.

    Are you sure the problem asks for the net force? The net force is given by Newton's 2nd law, which in this case gives: Fnet = m*v^2/r.

    The 1127 N force is the upward normal force exerted by the ramp on the skateboarder. Nothing fictional about it! (I assume the radius of the ramp is 7 m?)

    The way that the ramp forces the skater into circular motion is by exerting a very real force!

    You'll find the net force using Newton's 2nd law in all cases.
  4. Aug 2, 2010 #3
    well doc, thanks for the fast reply, good to see you are still around.

    anyway, yes the radius is 7m and the question does ask for the total force acting upwards.

    so you're saying that ontop of the mg force acted back on the skateboarder due to it's weight(597), the ramp exerts additional force to keep it in circular motion(530) hence the total force acted upwards is 1127? is this true cause you wrote that fnet is only the circular force which is 530, yet the anwer in my pdf says the total upward force is 1127N.
  5. Aug 2, 2010 #4

    Doc Al

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    Staff: Mentor

    No. The weight pulls down with a force of 597 N while the ramp pushes up with a force of 1127 N. The net force, given by mv^2/r, is 1127 - 597 = 530 N upward. This net force is called the 'centripetal' force.
    They must mean the total force exerted by the ramp, not the net force acting on the skateboarder.
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