# Zero position in Potential Energy

1. Feb 14, 2010

### new^2^physics

Hello Physics Friends,

Q: A 23 kg car runs straight off the side of a cliff and plummets into the water
which is located 10.0m below. If the car was moving at 12 m/s the
instant it left the cliff, determine the following:

I understand how to calculate the answer to this, which I have not listed here... BUT I need help with one thing....

My professor said "that it is important to realize that gravitational potential energy and kinetic energy will change as the car moves. Therefore it is imperative that you specify the location that is being dealt with in every location." Therefore he suggested to have a position A and B.

POSITION A: the car falling off the cliff
POSITION B: The car hitting the water

CAN ANYONE TELL ME WHY MY TEACHER CHOSE ZERO POSITION AT THE WATER SURFACE AND NOT THE CLIFF SURFACE??

Newtophysics

2. Feb 14, 2010

### willem2

It doesn't matter very much, you can choose 0 wherever you want. Most people would put the 0 of potential energy at the lowest point, so you don't have to deal with negative numbers.

3. Feb 14, 2010

### rcgldr

For a simplified model of gravity, where the force is constant regardless of distance from the source, the equivalent of gravity from an infinitely large disk or plane, then the potential energy difference between two points of different height = G m1 m2 (h1 - h0). For a simplified "absolute" potential energy, a common choice for a reference point is to set h0 = 0. Using "g" to represent the rate of acceleration of gravity on earth, it's common to use a simplified form of gravitational potential energy in the form m g h.

For a point source of gravity, potential energy difference between points is -G m1 m2 (1/h1 - 1/h0) = G m1 m2 (1/h0 - 1/h1). In this case the common choice for a reference point is to set h0 = , and all potential energies at finite distance from the point source are negative values.

For a infinite long line source of gravity (this would be more common with electrical charge, such as a wire), potential energy difference between points is G m1 m2 (ln(h1) - ln(h0)). In this case the common choice for a reference point is to set h0 = 1.

Still the choice of a reference point is arbitrary.

Last edited: Feb 14, 2010