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Find the potential gravitational and kinetic energy of a ball after 0.4 seconds

  1. Nov 14, 2011 #1
    1. The problem statement, all variables and given/known data

    A 3.0 kg metal ball is held 3.5m above the ground. Find the potential gravitational and kinetic energy of the ball after it is let go and has fallen for 0.4s


    2. Relevant equations

    EK= 1/2mv^2

    EP=mgh



    3. The attempt at a solution

    Okay I know how to find both the kinetic and potential energy, but how does the time effect it? Is there a certain equation I have to sub it in other than the ones given. Please help :)

    Potential= EP=mgh
    Ep=(3.0Kg)(9.8m/s^2)(3.5m)
    EP= 102.9 J

    Kinetic = EK=1/2mv^2
    EK=1/2(3.0kg)(9.8m/s^2)
    EK=14.7 J
     
  2. jcsd
  3. Nov 14, 2011 #2

    PeterO

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    Homework Helper

    Re: Find the potential gravitational and kinetic energy of a ball after 0.4 seconds..

    It would seem you actually don't know how to find KE.

    You have substituted an acceleration value in for the velocity.

    The 0.4 seconds is there as in that time the object will have fallen some distance [so you won't use 3.5 in the PE formula], and will have gained some speed - the value of which you will use in the KE formula.

    Normal kinematics for falling objects is used to find the distance and speed values.
     
  4. Nov 14, 2011 #3
    Re: Find the potential gravitational and kinetic energy of a ball after 0.4 seconds..



    Thank you for helping me realize that, I appreciate it. :D
     
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