Newton's Third Law
All forces in the universe occur in equal but
oppositely directed pairs.
There are no isolated forces; for every external
force that acts on an object there is a force of equal magnitude but
opposite direction which acts back on the object which exerted that external
force. In the case of internal forces, a force on one part of a system will
be countered by a reaction force on another part of the system so that an
isolated system cannot by any means exert a net force on the system as a
whole. A system cannot "bootstrap" itself into motion with purely internal
forces - to achieve a net force and an acceleration, it must interact with
an object external to itself.
 Hewitt's
CP Surf Unit I: Mechanics Unit 1:
Chapter
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