## anonymous one year ago The height, s, of a ball thrown straight down with initial speed 64 ft/sec from a cliff 80 feet high is s(t) = -16t2 - 64t + 80, where t is the time elapsed that the ball is in the air. What is the instantaneous velocity of the ball when it hits the ground?

1. phi

First, find the time "t" when the ball hits the ground (i.e. when s(t)= 0) by solving for "t" in $-16t^2 -64t+80- 0$ Next, find the derivative of the height with respect to t, $$\frac{d s}{dt}$$ This is the instantaneous velocity of the ball as a function of t. Evaluate this expression at the time found from the first step.