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Homework Help: How do I find mass in this equation?

  1. Jan 8, 2017 #1
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    1. The problem statement, all variables and given/known data
    A child’s toy that is made to shoot ping pong balls consists of a tube, a spring (k = 18 N/m) and a catch for the spring that can be released to shoot the balls. When a ball is loaded into the tube, it compresses the spring 9.5 cm. If you shoot a ping pong ball straight up out of this toy, how high will it go?

    2. Relevant equations
    I know how to solve this... one thing gets in my way: the mass. The solution says it's .0027 but I hardly find that plausible.

    3. The attempt at a solution
    I used f=kx and f = mg so mg=kx and m(9.81) = (18*.095m)
    My answer is like .17 not .0027.. any ideas?
    Last edited by a moderator: Jan 8, 2017
  2. jcsd
  3. Jan 8, 2017 #2


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    Staff: Mentor

    The problem does not ask for you to find the mass of the ping pong ball, it asks for you to find the height achieved by said ball. The problem statement should have provided the mass of the ping pong ball. Without it you cannot solve the problem.

    With missing data it is often possible to resort to a web search. If you google for the mass of a ping pong ball you'll find that the mass of a so-called official ping pong ball is 2.7 grams, or 0.0027 kg.
  4. Jan 8, 2017 #3


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    Science Advisor
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    Gold Member

    The solution says what is .0027? From gneill's post, i'm guessing it says that's the mass (in kg).
    What circumstance corresponds to equating those two forces? Think about it.
  5. Jan 8, 2017 #4


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    Homework Helper

    The problem text is wrong. It can not be the ball that compresses the spring by 9.5 cm. If the ball is compressed by the ball alone, the ball would never shot up.
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