Chapter 1.7, Problem 69E

Physical Science Use the height formula in Example 10 to work the given problems. Note that an object that is dropped (rather than thrown downward) has velocity $v_0=0$.

A ball is thrown upward from ground level with an initial velocity of 64 ft per second. In how many seconds will the ball be at the given height?

(a) 64 ft	(b) 39 ft
(c) Why are two answers possible in part (b)?

EXAMPLE 10 Physical Science If an object is thrown upward, dropped or thrown downward and travels in a straight line subject only to gravity (with wind resistance ignored), the height h of the object above the ground (in feet) after t seconds is given by

$h=-16t^2+v_{0}t+h_0$,

where $h_0$ is the height of the object when $t=0$ and $v_0$ is the initial velocity at time $t=0$. The value of $v_0$ is taken to be positive if the object moves upward and negative it if moves downward.