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Homework Help: Problem on centripetal acceleration and friction

  1. Oct 28, 2009 #1
    Suppose a nickel sits on a flat turntable that spins at 78 rpm (revolutions per minute) while sitting on a horizontal surface. The nickel is 15 cm from the center of the turntable. What is the minimum coefficient of friction that keeps the nickel on the turntable?

    Relevant equations: I am not sure which equations to use, to be honest. I know it involves calculating centripetal force and frictional force.

    This problem has stumped me all day. I believe it is unsolvable without knowing the mass of the nickel. I know a US nickel weighs 5 grams, but I do not know if we're supposed to assume that.
  2. jcsd
  3. Oct 28, 2009 #2


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    Homework Helper

    Hi vicviper89, welcome to PF.
    Identify the forces acting on the nickel. You have mentioned two. What is the third force?
    Write down the equations for centripetal force and the frictional force. To keep the nickel at rest, the net force must be zero. Using this condition find μ.
  4. Oct 28, 2009 #3
    Thanks for your help, I figured out that I had to set the two equations equal to each other and found out that the centripetal force is greater than the frictional force, which means it always slides. In effect meaning that the problem has no solution and my gut reaction was right.

    I have a new problem now:

    A Cannon shoots a cannonball with a mass of 3kg at a angle of 36 degrees with the flat ground. It has an initial velocity of 45m/s. The ball travels through the air with a cross-wind acting on it. The cross-wind applies a force of 200N to the ball acting at 90 degrees to the original direction of motion over the entire duration of flight. Can you determine where the ball landed?

    Once again, I have trouble figuring out which equations I need. Our formula sheet is poorly written and doesn't define any variables so I have no clue what means what.
  5. Oct 28, 2009 #4


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    Your explanation of the first problem is not clear.
    In the second problem, find the vertical and horizontal components of the velocity.
    Using the vertical component and acceleration due to gravity, find the time of flight.
    In the problem the direction of wind is not clear. Is it perpendicular to horizontal component of velocity?
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