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Tension in a String just before it breaks (Circular Motion)

  1. Oct 22, 2007 #1
    1. The problem statement, all variables and given/known data
    A 120 g ball on a 60 cm long string is swung in a vertical circle about a point 200 cm above the floor. The string suddenly breaks when it is parallel to the ground and the ball is moving upward. The ball reaches a height of 650 cm above the floor. What was the tension in the string an instant before it broke?

    2. Relevant equations

    3. The attempt at a solution
    I used the above equation to find the intial velocity when it breaks away from the string. Here's how I set that up:


    This yielded: Vi= 2.969 m/s
    Now my problem is how do I use this velocity to find the tension of the string just before it breaks?
  2. jcsd
  3. Oct 22, 2007 #2
    Draw the FBD and see what forces are acting on the ball and where the Tension fits in.

    also you're missing one important equation:

  4. Oct 22, 2007 #3
    Okay, so my FBD looks like this:


    So...the only force in the x direction is Tension. The force of tension is F=ma, so using the equation you gave me I get my a to be 14.691 m/s^2, then multipling by the mass I get the Force of Tension to be 1.76, but this isn't right. Is there some way I need to incorporate the weight into the equation, I'm sure there must be.
  5. Oct 22, 2007 #4
    Any help? Please?
  6. Oct 22, 2007 #5


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    Trust your FBD. The only force acting in the radial centripetal direction is the tension force. Check math and round off errors, problem statement, etc.
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