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Homework Help: How to find energy if mass and height is given?

  1. Oct 6, 2011 #1
    If you are going to throw a 1kg ball vertically upward and reach a maximum height of 18m. A)How much energy is needed?
    B)What should be the initial speed of the ball?

    My solution:
    mgh=1/2mv^2
    v=√2gh
    v=√2(9.8)(18)
    v=18.78m/s

    In getting the energy
    I derived the work energy theorem
    2w=vo^2
    2w=18.78^2
    w=176.34 joules
    I'm not sure if my answer is correct so I would be extremely grateful if someone could check it.

    Thanks.
     
    Last edited: Oct 6, 2011
  2. jcsd
  3. Oct 6, 2011 #2

    Hootenanny

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    Staff Emeritus
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    Gold Member

    Correct.
    By my reckoning, you are a decimal place off. It is likely a result of your rounding off the velocity.

    Note that instead of using the velocity, you could have simply used the fact that work = change in energy = mgh. Do you see?
     
  4. Oct 6, 2011 #3
    Thanks for the quick reply.

    Regarding on the fact that you had just said, that was actually my first solution but seeing that they both have the same result. I decided to use this one instead.
     
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