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Math Help - Average speed

  1. #1
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    Average speed

    please help me on the following problem:

    A rock released from rest to fall on a small airless planet falls y=gt^2 m in t sec, g a constant. Suppose that the rock falls to the bottomof a crevasse 20 m below and reaches the bottom in 4 sec.

    a) find the value of g.
    b) find the average speed of the fall.
    c) With what speed did the rock hit the bottom?
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  2. #2
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    Quote Originally Posted by macgyver View Post
    please help me on the following problem:

    A rock released from rest to fall on a small airless planet falls y=gt^2 m in t sec, g a constant. Suppose that the rock falls to the bottomof a crevasse 20 m below and reaches the bottom in 4 sec.

    a) find the value of g.
    b) find the average speed of the fall.
    c) With what speed did the rock hit the bottom?
    a) Substitute t = 4 and y = 20 into y = gt^2. Solve for g.

    b) Average speed = distance/time = .....

    c) t = 4 s, s = 20 m, u = 0 m/s, v = ?
    Substitute the data into s = (u + v)t/2 (one of the usual straight line motion under constant acceleration formula).

    Alternatively, t = 4, s = 20, a = g = answer to (a), v = ?
    Substitute the data into s = vt - at^2/2 (one of the usual straight line motion under constant acceleration formula).
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