Kristopher G. answered • 10/28/19

Phd Physicist and Mathamatician

This is a one dimensional kinematics problem. We know the time, acceleration, and distance covered. We want to know the initial velocity. Here we would use the first kinematic equation.

y=y_{0}+v_{0}t+(1/2)at^{2}

Since the rock hits the river we can consider the final position y=0, y_{0}=30 a=-9.8 m/s^{2} t=2.0s

simplifying this we can rewrite this as

0=y_{0}+v_{0}*t-(1/2)gt^{2} or v_{0}*t=(gt^{2})/2-y_{0}

Getting a common denominator we get

v_{0}t=(gt^{2}-2y_{0})/2 or v_{0}=(gt^{2}-2y_{0})/2t

Pluging in the numbers we get

v_{0}=(9.8*4-2*30)/(2*2)=-5.2m/s

or 5.2 m/s downward.