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Name: Belinda
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: 11/2/2005


Question:
I do not understand the reason behind choosing infinity as the point of zero gravitational potential energy.


Replies:
Hi Belinda,

The reason is that the gravitational potential energy for a spherical distribution of mass is given by:

U = GMm/r + constant

where the constant is arbitrary since only changes in potential energy are relevant. Obviously the simplest value for this constant would be zero. In that case, the potential energy goes to zero as r goes to infinity.

So to keep the equation as simple as possible, the potential energy is taken to be zero when r = infinity.

You could, of course, take the constant to be any other value. If, for example you choose constant = -GMm/R, where R is the radius of the earth, then the potential energy would be zero at the surface of the earth. Although that may seem to be a logical choice, calculations are simpler if the constant is taken to be zero.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Belinda,

The only reason is that it is convenient, easy to work with. Using infinity as the zero point results in a simple formula for gravitational potential energy. The equations end up easier to work with.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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