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Work/force/kinematics slingshot problem

  1. Mar 5, 2008 #1
    1. An average force of 8.2 newtons is used to pull a .4 kg rock, stretching a sling shot 43 centimeters. The rock is shot downward from a bridge 18 meters above a stream. What will be the velocity of the rock just before it hits the water? How much time will it take to hit the water.



    2. Relevant equations



    3. I just want to know if the spring/work equation, .5k(x)squared=w applies to the slingshot, and would stretching it by 43 CM = the x part of the equation? I don't know what to do once I get work. How do i get the velocity of the rock? I know A after shot would be 9.8, d is 18, but is final Velocity 0?
     
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  3. Mar 5, 2008 #2

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    It does apply, and yes. The spring PE is converted to KE. That's how you get the velocity. Final velocity nowhere is zero.
     
  4. Mar 6, 2008 #3
    Ok, so I put in the force*the distance it was pulled back to get the potential sling shot energy. I than set that equal to the KE equation and got V. Than I simply used that V as V0, used 9.8 as A, and 18 m as D, than solved for V1. Is this correct?
     
  5. Mar 7, 2008 #4

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    Absolutely. You find the time using the same kinematics.
     
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