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- Newton's third law is important as a check to verify that
- all forces have been included in the solving of a problem. An
- object resting on the ground exerts a force on the ground
- because of the force due to gravity. Since the
- object is not accelerating into the ground, however, there
- must be another force acting on the object. This force is the
- normal force, or reaction force, exerted by the ground on
- the object. In this case, since the object is at rest, the normal
- force must be equal in magnitude and opposite in direction to
- the weight of the object. The normal force is always
- perpendicular to the surface at the point of contact.
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