# A Roller Coaster Ride through Relativity/Appendix H

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## The Gravitational Potential at the surface of a star[edit]

The force of gravity on a mass *m* at a distance *r* away from a star of mass *M* is equal to:

The work done in pulling the mass from the surface of the star out to infinity is therefore:

This works out to be

and therefore the gravitational potential difference *Δφ* between the surface and ∞ is

Now by definition, the gravitational potential at ∞ is zero so the potential at the surface is negative, hence: