Homework Help: physics

Posted by elaine on Tuesday, October 19, 2010 at 8:54pm.

A stone is dropped from a 75 m high building. when this stone has dropped 15m, a second stone is thrown downward with an initial velocity such that two stones hit the ground at the same time. find a. time taken for the first stone to fall 75m. (b) the time taken for the second stone to hit the ground (c.) what as the initial velocity of the second stone

physics - Henry, Thursday, October 21, 2010 at 12:38pm
a. d = 0.5gt^2,
75 = 0.5 * 9.8t^2,
75 = 4.9t^2,
Divide both sides by 4.9:
t^2 = 75 / 4.9 = 15.3,
t = sqrt(15.3) = 3.9 s.

b. d = Vo*t + 0.5gt^2,
Vo*t = 15 m,
75 = 15 + 0.5 * 9.8t^2,
75 - 15 = 4.9t^2,
60 = 4.9t^2,
Divide both sides by 4.9:
t^2 = 60 / 4.9 = 12.2,
t = sqrt(12.2) = 3.5 s.

c. Vo*t = 15,
Vo * 3.5 = 15,
Vo = 15 / 3.5 = 4.3 m/s.

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