# Homework Help: Simple throwing ball up problem

1. Dec 15, 2007

### j_suder2

Simple throwing ball "up" problem

1. The problem statement, all variables and given/known data
Ball is thrown vertically with upward velocity of 18m/s when it reaches 1/4 of its maximum height above its launch point. Find initial launch velocity of ball.

2. Relevant equations
0.5mv2, mgh, v=d/t

3. The attempt at a solution

The 1/4 of the maximum height is throwing me off.
I tried using kinematics equations to first solve for the first part of the throw (the 1/4)
and then use final velocity of zero at the max height using deceleration of gravity to find this height.
I'm stumped, any help is appreciated.
Thanks.

2. Dec 15, 2007

### Staff: Mentor

I would do this with conservation of energy. Set your h=0 point at the 1/4 point (where v=18m/s)

3. Dec 15, 2007

### robphy

v is not generally equal to d/t.
While you can use energy conservation, in case you haven't gotten that far... or wish to use something more kinematical, you can use a formula (resembling the energy conservation approach) derived from the constant-acceleration position-vs-time and velocity-vs-time equations.

4. Dec 15, 2007

### j_suder2

Ok thanks, I found the height to be 32.8 m (?) at the quarter point, then set KE + PE final to = KE initial and got 25 m/s for an initial velocity. The answer is 21 m/s though. Not sure where I went wrong.

5. Dec 16, 2007

### kplooksafterme

Your height value is incorrect. I'd use conservation of energy to find the height (you don't need to set the height at the 1/4 mark to zero, I find this adds an unnecessary step; just set it equal to 1/4h). What is your energy conservation expression? Once you have the correct value of the height, it's merely a kinematics problem.

6. Dec 16, 2007

### rl.bhat

If h is the maximum height, you can wright two equation.
v1^2 - vi^2 = -2gh/4...(1)
0 - vi^2 =-2gh ....(2) solve these equation and find vi

Last edited: Dec 16, 2007
7. Dec 16, 2007

### kplooksafterme

rl.bhat, those equations are wrong... should be v1^2-vi^2 = -2gh/4....

8. Dec 16, 2007

### j_suder2

My conseravtion of energy expression is .5mv^2 + mgh = .5mv^2
This leaves me with two variables, so I am not sure what is next.
Thanks.

9. Dec 16, 2007

### Hootenanny

Staff Emeritus