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Homework Help: Help on this would be amazing; centripetal force & projectile motion

  1. Oct 16, 2007 #1
    1. The problem statement, all variables and given/known data

    David used a sling to kill Goliath. The mass of the rock was 2.5 kg and the sling had a length of 1.2 m. If it spun horizontally 2 m above his head 7 times in 3 seconds:

    a. What was the launch velocity?

    b. What is the centripetal force and tension?

    c. How far is David from Goliath?

    2. Relevant equations


    3. The attempt at a solution

    I tried setting ma=mv^2/r, but that would only give me the velocity it spins at, not the velocity at which it was launched. And I'm not sure what to do with the 2 m.

    Assistance would be great, I've been trying this for 2 hours and still nothing.
    Last edited: Oct 16, 2007
  2. jcsd
  3. Oct 17, 2007 #2
    7 times in 3 seconds gives you the angular speed, which will also be the launch speed. We assume it was rotating about David's head at a constant rate.

    From the angular speed you have calculated, you can get the centripetal force. This must equal the tension

    For c. you need to use the equations of motion, which is where the 2m comes in
  4. Oct 17, 2007 #3
    Thank you
  5. Oct 17, 2007 #4
    I am a little rusty, but I am quite sure that the launch speed does not equal the angular speed. The rock leaves the sling's cirular path in a direction perpendicular to the string.

    Angular speed=[tex]\omega[/tex] launch speed=[tex]v_{tangential}[/tex]

    Do you know of a relationship between angular and tangential speed?

  6. Oct 17, 2007 #5
    ah crap, yes, i meant the tangentil speed, lol.
  7. Oct 17, 2007 #6
    Yes i figured that out, but thanks for the lead. it helps a lot.
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