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Posted by on Tuesday, June 18, 2013 at 12:11pm.

A cement block accidentally falls from rest from the ledge of a 51.5 -m-high building. When the block is 12.9 m above the ground, a man, 1.60 m tall, looks up and notices that the block is directly above him. How much time, at most, does the man have to get out of the way?

  • physics - , Thursday, June 20, 2013 at 4:55pm

    V^2 = Vo^2 + 2g*d
    V^2 = 0 + 19.6(51.5-12.9) = 756.56
    V = 27.5 m/s.

    d = Vo*t + 0.5g.t^2 = 12.9-1.6 = 11.3 m
    d = 27.5*t + 4.9t^2 = 11.3
    4.9t^2 + 27.5t - 11.3 = 0
    Use Quadratic Formula and get:
    t = 0.385 s.

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