4f. The Third Law: Forces in Pairs
The third and final law of Newtonian mechanics is
as intuitive and enchanting as the statement "You
cannot touch without being touched." Yet the law
(shown below) is often misunderstood and
misapplied.
© Schlumberger Ltd., used with permission.
Earth and skydiver pull on one
another (down arrow) to overcome air
resistance (up arrow).
Newton's third law
If one object exerts a force on another object, the other object exerts an equal force
back on the first object.
According to legend, the Russian mystic Nostradamus's horse once refused to
pull a cart after being hit on the head by an apple. The horse reasoned that,
according to Newton's third law, any force that he exerted would be balanced by
an equal and opposite force. The cart would thus never move, no matter how
hard the horse tried.
A horse harnessed to a cart exerts an equal and opposite
force to the cart as it exerts a force against the ground.
Perhaps awed by the horse's remarkable ability to apply scientific laws from the
future (Newton developed his laws about a century after Nostradamus died),
Nostradamus failed to find the flaw in this argument. The mystic was forced to
get himself a less educated horse, which somehow did pull the cart.
The flaw in the first horse's argument is also a common misconception among
beginning physics students. Fortunately, it is easily remedied. The key is
understanding that action-reaction forces always act on different objects.
In the example here, the cart feels the unbalanced force applied by the horse
and therefore accelerates if this force is greater than the frictional force of the
ground on the cart. On the other hand, the horse feels an equal and opposite
force applied by the cart but is able to accelerate forward if the frictional force of
the ground on the horse is greater.
The two forces in an action-reaction force pair always act on two different objects.
Consequently, the two forces can never cancel each other out. Acceleration of a
particular object is caused by a net (unbalanced) force acting on that object.