Homework Help: physics

Posted by student on Monday, September 5, 2011 at 11:19pm.

A rock is thrown vertically upward with a speed of 14.0 m/s from the roof of a building that is 50.0 m above the ground. Assume free fall.

In how many seconds after being thrown does the rock strike the ground? in s

What is the speed of the rock just before it strikes the ground? in m/s

physics - Henry, Wednesday, September 7, 2011 at 7:35pm
Vf^2 = Vo^2 + 2gd,
d = 50 + (Vf^2 - Vo^2) / 2g,
d = 50 + (0 - (14)^2) / -19.6,
d = 50 + 10 = 60m above ground.

Vf = Vo + gt,
t(up) = (Vf - Vo) / a,
t(up) = (0 - 14) / -9.8 = 1.43s.

d = Vo*t + 0.5gt^2 = 60m.
0 + 0.5*9.8t^2 = 60,
4.9t^2 = 60,
t^2 = 12.24,
t(down) = 3.50s.

T = t(up) + t(down),
T = 1.43 + 3.50 = 4.93s

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