Kepler's Second Law
The shaded regions have equal areas. It takes equal times for m to go from A to B, from C to D, and from E to F. The mass m moves fastest when it is closest to M. Kepler's second law was originally devised for planets orbiting the Sun, but it has broader validity.
Source
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"OpenStax College, College Physics. April 28, 2014."
http://cnx.org/content/m42144/latest/?collection=col11406/1.7
OpenStax CNX
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