# Potential energy of an object with initial velocity.

1. Oct 11, 2009

### laxboi33

1. The problem statement, all variables and given/known data
A .300-kg ball is thrown vertically upward with an initial speed of 10.0 m/s. If the initial potential energy is taken as zero, find the ball's kinetic, potential, and mechanical energy at it's initial position.

2. Relevant equations
1/2mv(f)^2 + mgy(f) = 1/2 mv(i)^2 + mgy(i)

3. The attempt at a solution

I understand how to solve the problem. From my equation I've plugged in the 10 m/s for the inital and zero for the potential. Is potential energy zero because the book said it was? What I don't understand is why potential energy is zero when initial velocity is 10 m/s. If there is initial velocity shouldn't there be a numerical quality in the y direction? To be more clear, if an object is initially moving at a certain rate, shouldn't the ball have left the person's hand and given the ball some sort of height even a little bit?

2. Oct 11, 2009

### Delphi51

PE = mgy = 0 when y = 0 and the thing is, you can define y to be zero wherever you like. Sea level? Ground level? In this case, the height where it leaves the hand and when t = 0.