Solve Quadratics: Find Time for Rock Dropped from Washington Monument

[jubbly]

Homework Statement

A rock is dropped from the Washington Monument which is 555 feet tall. Using the formula h(t) = -16t^2 + volt + ho , find approximately how long it will take for the rock to hit the ground.

Homework Equations

h(t) = -16t^2 + volt + ho

The Attempt at a Solution

I'm doing the complete the square method
0 = -16t^2 - 9.8t + 555
-16t^2-9.8t=-555
-16t^2-9.8t +24.01 = -555-384.16 = -939.16
-16(t-4.9)^2 = -939.16
Divide both sides by 16
(t-4.9)^2 = 58.69
Take the square roots
t-4.9 = sqrt58.69

At this point I'm confused as to how to get a number for the time. If anyone can help it'd be a great! thanks.

 

[PhanthomJay]

Homework Statement

A rock is dropped from the Washington Monument which is 555 feet tall. Using the formula h(t) = -16t^2 + volt + ho , find approximately how long it will take for the rock to hit the ground.

Homework Equations

h(t) = -16t^2 + volt + ho

The Attempt at a Solution

I'm doing the complete the square method
0 = -16t^2 - 9.8t + 555
-16t^2-9.8t=-555
-16t^2-9.8t +24.01 = -555-384.16 = -939.16
-16(t-4.9)^2 = -939.16
Divide both sides by 16
(t-4.9)^2 = 58.69
Take the square roots
t-4.9 = sqrt58.69

At this point I'm confused as to how to get a number for the time. If anyone can help it'd be a great! thanks.

You erroneously put in vo = -9.8, when in fact vo = 0. I think you got velocity (v) and acceleration (g) mixed up. (Incidentally, if you were looking at acceleration, in the ft/sec/sec units, g is 32 ft/sec/sec. That's where the 16 comes from, it was built into the equation already, per the 1/2at^2 term of the kinematic equation).