Interactions
give rise to forces
(Newton's 3rd law)
"To every action there is an equal
and opposite reaction."
Newton's 3rd law of Motion deals
with the way objects interact with each over.
When you hold a weight in your hand, for example,
the weight pushes down on your hand and your
hand pushes up on the weight with an equal and
opposite force. If you take your hand away,
the weight will fall the floor, accelerating
due to the gravitational force.
In Newton's
own words
(To be read only for historical insight. The
mixture of old English and terminology can confuse!)
To every action there is always opposed
an equal reaction; or the mutual actions of
two bodies upon
each other are always equal and directed to
contrary parts.
What ever draws or presses another is as
much drawn or pressed by the other. If you press
a stone with your finger, the finger is also
pressed by the stone. If a horse draws a stone
tied to a rope, the horse (if I may say so)
will be equally drawn back towards the stone.
"Principia - 1726"
Rockets
and the 3rd law
A rocket is a spectacular example of Newton's
3rd law, where hot gases produced
in the rocket's combustion chamber push outwards
in all directions on the sides of the chamber.
If the chamber were completely closed on all
sides the rocket would not operate. However
because there is an opening at the rear of the
chamber, the expanding gases cannot exert a
force in the direction of the opening. Therefore
the unbalanced force on the front of the chamber
can thrust the rocket forward and up.
The rocket simultaneously exerts a force
on the gas which expels the gas from the rocket
engine. The force of the rocket on the gas and
the the force of the gas on the rocket are equal
in magnitude but opposite in direction.
Measuring
equal and opposite forces
A
simple way of demonstrating the working of Newton's
3rd law is to join the
hooks of two spring balances together and pull
the two ends apart. When pulled apart they will
both register the same magnitude of force, but
in opposite directions. Whether both ends are
pulled or one end is attached to a wall and
the other spring pulled, the two springs will
register equal and opposite forces.