Download Newton`s Third Law of Movement

Newton's Third Law of Movement
Every action has an equal and opposite reaction.
In this formulation action means action of force. It sounds simple, but it is a little
“tricky” and understanding it requires some intellectual effort. Let's consider a few
examples.
(a) We push the wall – the wall does not change its position.
(b) We stand on the floor, our body pressed on the floor – the floor does not move.
(c) A heavy TV set stands on a table – the table does not move.
Hundreds of such examples can be found. A force is acting on “something” and this
“something” does not change its state of motion as required by Newton's Second Law
of motion. Why?
Let's explain in more details the situations in the listed examples.
Pushing means acting with force on the wall and, according to Newton's Third Law of
Motion, the wall exerts a force on the pushing body.
This force is called a reaction. Both forces, the one of action and of reaction are equal
in magnitude and oppositely directed. The stronger we push, the stronger the reaction
is, so the resultant force is equal ZERO and there is no acceleration or deceleration
neither of the body pushing the wall nor of the wall. Analogical explanation is true for
all examples listed above.
If we push a wall, the action force is exerted on the wall, the reaction force
“produced” by the wall is exerted on our hand. We have two forces, equal in
magnitude and oppositely directed, but acting on two different objects – wall and
hand. In such a case we can say: the resultant of these forces is equal to zero. The
resultant force not net force.
Then can we talk about net force.Please look at Fig.1.
Forces A and B on Fig.2 act on the same object. They may be exerted by two playing
children, for example. They are completely independent of each other, but they act on
the same object. The sum of these forces is a net force acting on the object.
The sum of the force C of gravity of the blue object and of the reaction force D is a
resultant force as these forces act on two different objects. Force C acts on the floor
and force D on the object lying on this floor.
Once we are familiar with the Newton Third Law of Motion we can proceed, or rather
go back to the Second Law. While reading the paragraph explaining the Second
Newton's Law you will understand why we started with the Third Law.