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Homework Help: Another SUVAT problem

  1. Dec 5, 2015 #1
    1. The problem statement, all variables and given/known data
    a) How much time does a dropped weight take to fall 120m down a cliff? (I answered this correctly as 4.9s -- √(s/0.5 x g).)
    b) What would the time be (in question a) if the weight was thrown downwards at 2.5m/s?

    2. Relevant equations

    3. The attempt at a solution
    None, apart from a mistaken rearrangement of s = ut + 0.5at^2.
  2. jcsd
  3. Dec 5, 2015 #2


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    I am not familiar with SUVAT, could you provide more detail?
    In your equation S = Ut+ .5At^2, the only thing that changes from your first problem is that now u is not 0.
    You will end up with a quadratic equation for t. Pick the positive solution.
  4. Dec 5, 2015 #3
    How would I isolate t?
  5. Dec 5, 2015 #4


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    Have you ever solved a quadratic equation?
  6. Dec 5, 2015 #5


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  7. Dec 5, 2015 #6
    My bad, I was trying to rearrange for t. This crossroad is new to me.

    120 = 2.5t + 4.9t^2
    4.9t^2 + 2.5t - 120 = 0

    A quadratic formula later... 4.7s. Correct! Thanks all.
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