An arrow is shot straight up in the air at an initial speed of 15 m/s. After how much time is the arrow heading downward at a speed of 8 m/s?

2.35 seconds

Explanation:

Remark

This is a question where direction matters.  Let us call down + and up minus. It won't matter. The answer will be the same.

Formula

a = (vf - vi)/t

Givens

a = 9.8 m/s^2

vi = - 15 m/s

vf = 8m/s

Solution

9.8 = (8 - - 15)/t                 Multiply both sides by t

t * 9.8 = 23                       Divide by 9.8

t = 23/9.8

t = 2.35 s

conservation of mass

mass before something happens = mass afterwards