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Math Help - Acceleration/Velocity Help

  1. #1
    chris_needshelp
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    Acceleration/Velocity Help

    A child drops a ball from a bridge. The ball strikes the water under the bridge 2.0 seconds later. What is the velocity of the ball when it strikes the water?

    ^ Is the question. I have no idea how to calculate that, because there isonly one variable.

    Please help, this is due tomorrow.
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by chris_needshelp View Post
    A child drops a ball from a bridge. The ball strikes the water under the bridge 2.0 seconds later. What is the velocity of the ball when it strikes the water?

    ^ Is the question. I have no idea how to calculate that, because there isonly one variable.

    Please help, this is due tomorrow.
    One thing we should do first is set up a coordinate system. So for the record I've got an origin (which we really won't need) at the level of the water, and I've got a positive y direction pointing upward.

    You've got more information than you might think you do. First, we know the initial velocity of the ball: it's 0 m/s because the child is dropping the ball, not throwing it. We also know the acceleration of the ball: 9.8 m/s^2 downward, or in our coordinate system a = -9.8 m/s^2.

    What equation do we have that contains v0, v, a, and t?

    v = v0 + at

    v = 0 -9.8*2 = -19.6 m/s

    So the velocity is 19.6 m/s downward when the ball strikes the water.

    A final quick note: The acceleration of the ball is a = -9.8 m/s^2. g = 9.8 m/s^2. g is NEVER negative. This is a common notation mistake.

    -Dan
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