Potential energy of an object with initial velocity.

[laxboi33]

Homework Statement

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.

Homework Equations

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

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?

 

[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.

 

[tomcue]

The potential energy at the initial position is often taken as zero for convenience and simplicity in calculations. This does not necessarily mean that the ball has no height or displacement at the initial position. It simply means that we are choosing to measure the potential energy relative to a reference point where the potential energy is zero. In this case, the reference point is likely chosen to be the ground level or the hand from which the ball is thrown. Therefore, the potential energy is not necessarily zero, but rather it is taken as zero for the purpose of solving the problem. It is important to note that potential energy is a relative concept and can be chosen at any point as long as it is consistent throughout the problem.