Newton’s Third Law of Motion
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.
But… consider such example – Fig 1
On a thin, long wooden rod there is suspended tray. We put one iron ball on it. In this case – A, on Fig.1 the force of gravity exerted on the ball is our action. The elasticity of the wooden rod creates a reaction. The rod is strong enough to support the ball.
In case B we have two iron balls, wooden rod is bent, but still is able to “produce” the force of reaction equal to the acting force.
Case C may be wrongly interpreted as an invalidation of Newton’s Third Law of Motion – that there was an action force, but no reaction force. Nothing more untrue. If the body which was supposed to create a reaction (the wooden rod) was to weak, it simply fractured (broke). Now the three balls lie on the floor and the floor is “producing” the reaction force.
At this point is it worth to distinguish two very close, but different terms.
Resultant force and net force.
The difference is very subtle, but important to understanding many problems connected with Newton’s Laws of Motion. We will cite here a few definitions for these terms which you can read after entering in Google define: resultant, and then define: net force.
gives (among other definitions):
Vector sum of two or more vectors,
A single force that produces the same result that all forces would if acting together,
The single force that has the same effect as two or more forces applied simultaneously at the same point.
define: net force
gives (among other definitions):
sum of the forces acting on an object,
the vector sum of all forces acting simultaneously on an object,
the sum total of all forces acting upon an object.
The subtle difference between resultant force an net force lies in the parts of the definitions written in red color.
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.
When then can we talk about net force? Look at Fig.2.
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.