Homework Help: Physics

Posted by Josh on Monday, September 11, 2006 at 11:40pm.

A rock is thrown vertically upward with a speed of 11.0 m/s from the roof of a building that is 70.0 m above the ground. Assume free fall.

In how many seconds after being thrown does the rock strike the ground?
GOT THIS!!!! t=5.07s

What is the speed of the rock just before it strikes the ground?
I HAVE NO IDEA HOW TO DO THIS!!!

Let's check this
s(t)=(-1/2)gt^2+v_0t+s_0
s_0=70m, v_0=11m/s, g=9.8m/s
We want t for s(t)=0
0=(-1/2)9.8t^2+11*t+70
Solving for t 0=-4.9t^2+11t+70
t= -11/9.8 +/-sqrt(121 - 4*(-4.9)70)/-9.8
t = 5.07 so the time is correct
v(t)=-gt+v_0 at t=5.07 v=-9.8*5.07+11=-38.7m/s downward

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