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How tall is the bridge?

  1. Jan 14, 2015 #1
    1. The problem statement, all variables and given/known data
    A rock thrown horizontally from a bridge hits the water below 30 meters away in the horizontal direction. If the rock was in the air for 2 seconds, how tall is the bridge?

    2. Relevant equations
    d=.5gt2


    3. The attempt at a solution
    I think I am going about this the right way. Just want to double check.

    d=.5*9.81*4
    d= 19.62m
    The bridge would be 19.62m high?

    cheers,
    Keith
     
  2. jcsd
  3. Jan 14, 2015 #2

    phinds

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    It's been about 50 years since I've actually worked out a problem like that but it sounds wrong, and here's why: the acceleration due to gravity is 9.8 meters per second per second. That means that the rock will travel 9.8 meters in the first second, but much more in the next second, so the total can't be 9.8 x 2 which is what you got.
     
  4. Jan 14, 2015 #3
    It looks right to me. The first second it travels ~ 5 m, and the second second, it travels ~15 m.

    Chet
     
  5. Jan 14, 2015 #4

    phinds

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    Guess I have it wrong then that it travels 9.8m in the first second. I guess what the figure means is that at the end of the first second it is MOVING at 9.8 m/s, not that it has MOVED 9.8 m ... 50 years ago I bet I would have got it right :oldtongue:
     
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