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Homework Help: Circular Motion Fun!

  1. Nov 16, 2004 #1
    Here's the Question :

    A 5.00-kg mass is on the end of a massless rigid rod of length l=70.0cm which pivots freely about the other end. The mass is moving in a verticle circle. When the rod makes an angle of theta=55.0° with the horizontal, the speed of the mass is 1.60 m/s.

    a) calculate the magnitude of the force exerted on the mass by the rod.

    I already calculated this to be 21.9N and this is correct.

    b) (WHERE I NEED HELP) Calculate the speed of the mass at the highest point of the circle.

    Now I know I need to write the forces for this point... F = mv²/L - mg

    but this equation gives me 2 unknowns, where do I get a value to sub for F (the force exerted by the rod)...or is there some simplifying step I'm missing here...

    please help,
  2. jcsd
  3. Nov 17, 2004 #2


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    Science Advisor

    I assume that the first part specifically asked for the force exerted on the mass by the rod AT 55 degrees above the horizontal?

    At the mass's highest point the rod is vertical and its force on the mass is vertical. There is a force of gravity downward on the mass but it is not moving upward or downward so the force of the rod on the mass must be exactly equal to the force of gravity on the mass.
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