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Newton's Third Law of Motion


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Sir Isaac Newton

Sir Isaac Newton[1]

Born: December 25, 1642 in Lincolnshire, England

Died: March 31, 1727 in Kensington, London

Sir Isaac Newton was one of the greatest and most influential English mathematicians and scientists of his time. He laid the foundation for differential and integral calculus. His work on optics and gravitation made him one of the greatest scientists in the world. His three laws of motion helped advance the study of classical mechanics as we know of it today.

Isaac Newton propounded the three laws of motion. His third law of motion deals with actions and reactions.


The Objectives of this module are to enable the teachers to :-

  1. improve their instructional design.
  2. apply constructivist approach in designing lessons on science.
  3. inculcate an insight on common pitfalls in comprehension of the significance of Newton's Third Law of Motion in Our Day-to-Day experiences



Let us consider the following events:

  1. A magnet and a piece of iron of a different mass than that of the magnet, are placed in a short distance apart over a relatively smooth surface where these bodies are free to move. What do we observe when these bodies are let free to move?
  2. Sitting over skateboard, placed over a smooth surface, a football is thrown in the forward direction. What is observed the moment it is thrown?
  3. An air pistol is fired at an inflated balloon target. What is felt in our hand the moment the trigger is pulled?

Let's watch these simple but elegant clippings to illustrate a fundamental phenomenon:


Can we identify any commonality among the occurrences? We can see that in each of the above cases two bodies are involved in the course of actions that follow. Further, both these bodies are involved in simultaneous activities - albeit in different external manifest. Keeping these in view - and pondering over them as well - we move on to our next discussion.

Concept of Force

We all know what a force is, don't we?

Force is an agent which changes, or tends to change the state of rest or of uniform motion of a particle in a straight line.

With this definition in mind, can we identify a few forces whose effect we see naturally around us?

Newton's Third Law of Motion

Perhaps we are all familiar with the wordings for Newton's third law:

For every action there is an equal and opposite reaction.

This statement is fairly well known to us, but there is a difference between memorizing the words and really understanding what they mean. These words are to be truly understood in all their perspectives.

Action-Reaction Pairs

We all know that Earth orbits the Sun because a gravitational force acts between them, so try this question. Which is greater the gravitational force of the Sun on Earth - or - the gravitational force of Earth on the Sun? If you thought they were exactly the same, congratulations. If not, let us look at this question using Newton's third law.

The action and reaction in Newton's third law are forces. Every force in nature comes in a pair, so to say. The Sun's gravitational force acting on Earth is an action. The reaction is the Earth's gravitational force acting on the Sun. Earth's gravitational force on the Sun and the Sun's gravitational force on Earth are an action-reaction pair and exactly equal in magnitude.

We may view the occurrences in the video clips below:

Can we organise some such simple yet elegant activities for the learners?

  • A Common Pit of Misconception ...

A pair composes of two - so an action-reaction pair here implies two forces. If the Sun acts on Earth, the reaction is Earth acting on the Sun with an exactly equal but opposite force.

This part is often confusing: not all equal and opposite forces form an action-reaction pair as mentioned in Newton's third law.

If you weigh 120 pounds that means that Earth's gravity is pulling you down with a force of 120 pound-weight. If you are standing on a solid floor, you do not accelerate downward because the reaction force of the floor is holding you up. The floor exerts an upward force of 120 pound-wt. on you. These two forces - your weight acting downward and the reaction of the floor acting upward - are equal and opposite, so you remain at rest on the floor.

Do these two equal and opposite forces constitute an action-reaction pair? Let's look at the problem a bit more closely. The forces are caused by the Earth on you and by the floor on you. There are three objects involved in this system instead of two. So these forces are not a Newton's third law action-reaction pair. These are two different forces acting on you which happen to be equal and opposite.

  • ...And A Way Out From It

The Newton's third law reaction to Earth pulling you down with a 120 pound-wt. force is that with which you pull the Earth up with a 120 pound-wt. force. The forces, Earth on you and you on Earth, form an action-reaction pair.

Now perhaps we can locate many such occurrences around us which involve an action-reaction pair. How about considering the following to be taken up as school/classroom activities by adapting according to our social perspectives and available resources:

Let us now ponder over few brain-teasers related to our topic in hand:

1. In a cart-bullock system as the bull pulls the cart, the cart also, in turn, pulls the bull with an exactly equal and opposite force. Why is it then the cart moves toward the way the bull pulls it, and not the other way round?

Hint: While thinking over it, just try to discern the two situations:

a. when the cart-bullock system is an isolated one, and

b. when the cart-bullock system is on a ground with a moderate coarseness of surface (thereby producing a frictional resistance force).

2. How at all a motion comes into being in the Universe? Every force would have

been neutralised by its equal & opposite "reaction" and thereby rendering a zero

effect on everything else!? Think...

Hint: Well here also we have to synchronise the concepts like system, isolated system, connected

system, internal forces, external forces, etc. And of course, never to forget the maxim that:action and reaction never act on the same body.

Jyotipriya Bhattacharya

3.In the above diagram (Fig. 1 and 2) an object (a stick) is shown to be inserted in a liquid in beaker. As the system is shown in the diagram, which way will the pointer of the scale go after the the stick is inserted, if we assume that the scale was perfectly balanced before the stick was inserted? And why?

Hint: The liquid exerts buoyant force on the stick, and the stick in turn ... ? Think.

An Activity (In Bengali)

This is a simple activity, described in Bengali, one of the official languages in West Bengal, and in which students of most of the schools in our state study. Teachers may take up the cue in organising many more of such activities in the school/classroom transactions. Here it goes:

This is supposed to be a flash video. You'll need the flash plugin and a browser that supports it to view to it.

Jet Propulsion - Simplified

Expected Outcomes

Teachers after going through the above design the teachers will be able to:

1.Improve their instructional design on Newton's Third Law of Motion.

2. To incorporate the Constructivist approach.

To Get Informed Further

One can go to the given links to know more about Newton's Third Law of Motion:

You may find some interesting discussions in this forum: › ... › Advanced Physics Forum.

Our Team

  • And We Are The Ones Who Worked On This Module

Gautam Bhattacharya
Md. Quamruzzama
Tarun Ghanti
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