Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Circular motion I HAVE NO IDEA WHAT I AM DOING

  1. Jan 14, 2010 #1
    1. The problem statement, all variables and given/known data
    A ball of mass M attached to a string of length L moves in a circle in a vertical plane as shown (picture of circle with a radius drawn L and M as a ball on the circle.) the tension at the top in the string is twice the weight of the ball. At the bottom the ball JUST clears ground Air resistance is negligible. Express your answers in terms of M,L, and g.

    a. determine the magnitude and direction of the net force on the ball when it is at the top.


    b. Determine the initial velocity of the ball at the top.

    c.Determine the time it takes the ball to reach the ground if the string is cut when the ball is at the top of the circle.

    d. Just before the string was cut, what was the string tension at the bottom? Assume that the ball has the same velocity at all points around its path.

    2. Relevant equations

    3. The attempt at a solution

    Sorry I didn't show too much work but that is because i truly don't know where to start. PLEASE HELP
  2. jcsd
  3. Jan 14, 2010 #2


    User Avatar
    Homework Helper

    At the top of the circle, what are the two forces acting and what direction are they in? The resultant of the two forces give the net force.

    Yes you will need to use mv2/r. When you get the answer for part one, you can use mv2/r to get 'v'.

    You'll get 'v' from above. Just use the parabolic equations of motion to find time,t. Hint: When the string is cut, the ball flies off at a tangent to the circle.

    Form the equation for the resultant force at the bottom and you will have 'v' from before.
  4. Jan 14, 2010 #3
    for part b though that formula solves for v I need Vo.
  5. Jan 14, 2010 #4


    User Avatar
    Homework Helper

    I think part b just wants you to get v
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook