Newton's Third Law of Motion
Click
for an introduction to forces
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Newton's third law states
Whenever one object exerts a force on a second object, the second
object exerts an equal and opposite force on the first object.
Simply stated
"To every action there is always an equal and opposite reaction"
One force we call the action
force and the other we call the reaction force.
It does not matter which force we call action and which we call reaction.
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| It is important
to note that all forces exist in pairs. We may not be aware of action and
reaction forces around us but they happen even during the simplest act of
walking. We push against the road surface with our shoes and the road pushes
back with an equal force but in the opposite direction to the force that
we exert on the road. It is the force the road exerts on us that causes
us to move forwards. Similar to a bike tyre spinning in contact with the
road, the tyre exerts a force on the road and the road exerts an equal force
in the opposite direction pushing the bike forwards. A swimmer moves through
the water only because he pushes water backward and the water pushes the
swimmer forwards. |
| A rocket pushes gases out its
exhaust nozzle. The force of the gases escaping the engine produces an equal
and opposite force that accelerates the rocket skyward. |
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| The recoil of a rifle or a cannon
is due to Newton's Third Law of Motion. The force the cannon exerts on the
cannon ball accelerating it out from the barrel is equal and opposite to
the force the ball exerts on the cannon to recoil it backwards. |
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All missiles obey Newton's
Third Law of Motion. Fuel is rapidly burnt to produce hot gases that
expand quickly, escaping with a great force from the nozzle. The rocket
pushes gases out the back and the gases push the rocket forward.
Click
to see an exocet missile in flight.
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| Propellers are designed to push
air backwards. The force with which the plane pushes air back is equal in
magnitude and opposite in direction to the force the air pushes the plane
forwards. |
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It is exactly
the same with propellers that power ships and submarines. The difference
is that water, rather than air, is pushed backwards. |
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