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Can I always assume Yi is 0m?

  1. Feb 1, 2017 #1
    1. The problem statement, all variables and given/known data
    A man builds a slingshot that can launch water balloons. If he launches a water balloon at 45.0 degrees to the horizontal at a speed of 35.0 m/s, how far away will the balloon hit?

    2. Relevant equations
    xf=xi + vix t
    yf=yi + 1/2(viy+Vfy)t
    yf=yi + viyt + 1/2at^2
    vfy2 = viy2a + 2ay

    3. The attempt at a solution:

    ti = 0
    yi = 0 (CAN I ASSUME THIS?)
    viy = 24.75 m/s
    vix = 24.75 m/s
  2. jcsd
  3. Feb 1, 2017 #2


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    As long as you are consistent with the interpretation of the other given quantities, yes you can.
  4. Feb 1, 2017 #3
    I just assumed that y2 = 0 because it hits the ground. Therefore, I assumed because he was holding the slingshot y1 couldn't equal 0.
    Therefore if y1=0, then y2= a negative value because it's lower.
  5. Feb 1, 2017 #4


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    Is the balloon launched at some height above ground? What is that height?
    When you write
    yi and yf need to be measured from the same point. That's what what I meant by "consistent".
  6. Feb 3, 2017 #5


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    The problem statement says nothing about the launch or landing height so you have to assume they are the same.
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