How do tidal forces differ from gravitational ones?
Tidal forces are produced when the gravitational force affecting one part of a
body...say the part closest to the center of the gravitational force...is
stronger than at a more distant part farther from the gravitating body. This
produces a net 'differential' force or stress across the body which can cause
it to deform and raise 'tides'. If the tidal force is stronger than the forces
holding the body together, it will break apart. This defines the Roche
distance. The Moon is currently about 10 times farther from the Earth than its
Roche distance, but when it was first formed it may have been at nearly this
For more information of the mathematical kind, visit
Eric W. Weisstein and Wolfram Research, Inc. page on calculating the Roche Limit.
Copyright (C) 1997 Dr. Sten Odenwald
Ask the Astronomer.