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Homework Help: Acceleration - all the energy is gravitational what speed was the ball thrown at?

  1. May 11, 2012 #1
    Acceleration -- all the energy is gravitational what speed was the ball thrown at?

    1. The problem statement, all variables and given/known data
    A ball is thrown up in the air and reaches a height of 2.2m assuming that all the energy is gravitational what speed was the ball thrown at.

    2. Relevant equations
    I cannot find the equation

    3. The attempt at a solution
    ??any help would be appreciated
  2. jcsd
  3. May 11, 2012 #2
  4. May 11, 2012 #3
    Re: Acceleration?

    1. Well you are given the height it reaches. You also know the acceleration due to gravity. Finally you know the velocity of the ball at maximum height. So you know Smax, a and v. You want to find u (the initial velocity)

    2. You must use the relevant 'SUVAT' equation.
    You can use v^2=u^2 +2as
    ..to rearrange to find u.

    Hope that helps.
    p.s. Just to let you know, I'm from the UK, am not sure if they teach it the same in the US assuming you are in the US? but i suspect this applies universally.
  5. May 11, 2012 #4


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    Science Advisor
    Homework Helper

    welcome to pf!

    hi james! welcome to pf! :smile:

    as a matter of interest: since this is an energy question, why have you titled it "acceleration" ? :confused:

    anyway, show us your energy equations :smile:
  6. May 11, 2012 #5
    Re: Acceleration?

    He doesn't know about conservation of energy yet. This is a week-one physics problem meant to be solved using the one-dimensional motion equations.

    The equations you're looking for are at the bottom of this page.


    edit: then again, the question does mention energy so maybe I'm wrong.
  7. May 11, 2012 #6
    Re: Acceleration?

    This is a very poorly worded question, because it doesn't tell you from what height the ball is thrown. It could have started at -713 meters for example. Most people will presuppose a beginning point of 0 meters. But you have to be careful making that presupposition since it could result in a wrong answer in other problems (whereas in this problem is just makes it solvable). You should probably explicitly state the assumption that the ball begins at 0 meters.

    And it's been brought up, but let me ask you directly: What are you learning in your class right now (or in what context are you working this problem [it may not be for a class])? Depending on that context, the approach to help you changes. As others have said, it matters if you (should) know conservation of energy or if you're working with kinematic equations.
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