Newton's Third Law of Motion
TOPIC: Force and Motion
DESCRIPTION: A set of mathematics problems dealing with Newton's Laws of Motion.
CONTRIBUTED BY: Carol Hodanbosi
EDITED BY: Jonathan G. Fairman - August 1996
The third law of motion states that if a body exerts a force on
a second body, the second body exerts a force that is equal in
magnitude and opposite in direction to the first force. So for every
action force there is always a reaction force. No force can occur
The book lying on the table is exerting a downward force on the
table, while the table is exerting an upward reaction force on the
book. Because the forces are equal and opposite, the book remains at
rest. Notice also that the table legs are in contact with the floor
and exert a force downward on it, while the floor in turn exerts an
equal and opposite force upward.
Questions for you to consider:
- If forces are always equal and opposite in action and
reaction, how is it possible for an object to accelerate?
- Explain, in detail, using the third law of motion, how a
person is able to walk forward.
- There is a classic problem that physicists like to ask
students. A horse is pulling a carriage on a level ground. The
horse knows the third law of motion. He tells the carriage that he
will exert a force forward, and the carriage will exert a force
equal to the horse's force but in opposite directions. Therefore,
the horse explained, he can never pull the carriage forward. Can
you explain to the horse that he is mistaken? How is he able to
pull the carriage forward?
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