Kinematics (a little more difficult then I am used to)

[Roger Wilco]

Homework Statement

Ball A is released from rest at height of 40 ft at the same time Ball B is thrown upward from 5 ft off the ground.

If the Balls pass one another at 20 ft above the ground, determine the speed at which ball B was thrown upward.

Homework Equations

$v=v_o+at$
$v^2=v_i^2+2a(y-y_i)$
$y-y_i=v_it+.5at^2$

The Attempt at a Solution

I have used the information about ball A to determine the time it takes A to reach a height of 20 ft:
Choosing down as positive, $20=16.1t^2$

thus, t=1.115 s. Now this must be the same time that B reaches 20 ft. I know that g is pulling down on it and it must cover a vertical distance of 15 ft in 1.114 s.

I am just getting stuck in trying to write that last sentence algebraically.

~RW

 

[Midy1420]

You're on the right track. You are looking for the initial velocity of ball B. We know the displacement (15ft), we know the acceleration (-32.2 ft/(s^2)) and you found the time it takes to travel this distance (1.115 s) it takes to reach that value. Which equation would you chose, to solve for Vinitial?

 

[Roger Wilco]

Oh good grief. I feel like such a mutton-head (is mutton-head hyphenated?).

Thanks Midy,
~RW