1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: 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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook