Chapter 4.8, Problem 20P

To determine

To find how much later will it hit the ground ?

Answer to Problem 20P

10 seconds later the ball will hit the ground .

Explanation of Solution

Given information: A ball is thrown upward from the top of a 98-meter tower with an initial speed of 39.2m/s.

Calculation: The required solution is obtained as,

Use the projectile equation, $h=vt-4.9t^2$ .

Velocity , $v=39.2m/s$ .

Height, $h=-98m$

  $\begin{array}{l}h=vt-4.9t^2\\ -98=39.2t-4.9t^2\\ 4.9t^2-39.2t-98=0\\ t^2-8t-20=0\\ \left(t-10\right)\left(t+2\right)=0\\ t=10,-2\end{array}$

Ignore negative value.

  $t=10$

Hence, 10 seconds later the ball will hit the ground .