Newton’s Third Law of Motion: Explanation, Statement, Examples

Newton’s Third Law of Motion is one of Sir Isaac Newton’s Laws of Motion that regulate the motion of objects. Many of us refer to this rule as the “action-reaction” law, and it looks to most of us to be quite simple. This couldn’t be further from the truth. This seemingly simple concept is difficult to grasp and use in real-world situations. So, with the assistance of this essay, let’s start delving and understand this law.

What is Newton’s Third Law of Motion?

Newton’s Third Law of Motion is one of the three Newton’s Laws of Motion so let us first refresh our knowledge about the Laws of Motion and then further take the specific case of Newton’s Third Law of Motion in detail.

Newton’s First Law of Motion: According to this law, if no net force acts on a body, then the velocity of the body cannot change; that is, the body cannot accelerate.

Newton’s Second Law of Motion: According to this law, the net force on a body is equal to the product of the mass of the body and the acceleration of the body.

Newton’s Third Law of Motion: According to this law, when two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction.

Hence, Newton’s First Law of Motion describes force, Newton’s Second Law of Motion gives the measurement of force, and Newton’s Third Law of Motion relates the forces between two interacting bodies.

Newton’s Third Law of Motion tells us about the nature of the force, what happens when a body exerts the force on the other body, what are the directions of the forces during the interaction of two bodies as well as what will be the magnitude of the forces in relation to each other. Let us check out the answer to these questions in the coming passage.

Newton’s Third Law of Motion Explanation

Newton’s Third Law of Motion connects the forces exerted by the bodies on one another. Let us consider the case of two objects: object \(A\) and object \(B\). If object A exerts a force \({F_B}\) on object \(B\), then object \(B\) also exerts a force \({F_A}\) on object \(A\). The forces are named \({F_B}\) and \({F_A}\) based on the object for which they bring the change in the motion. The force \({F_B}\) may change the momentum of object \(B\) and the force \({F_A}\) may change the momentum of object \(A\).

According to Newton’s Third Law of Motion, when object \(A\) exerts the force \({F_B}\) on object \(B\), object \(B\) in return exerts a force \({F_A}\) on object \(A\). The force \({F_B}\) is the action and the force \({F_A}\) is the reaction. Both the forces \({F_B}\) and \({F_A}\) are equal in magnitude but opposite in direction. If the force \({F_B}\) acts towards object \(B\), then the force \({F_A}\) acts towards object \(A\) that is in the direction opposite to the direction of force \({F_B}\). So, the directions of the forces \({F_B}\) and \({F_A}\) are opposite to each other. Mathematically it can be written as

\(\overrightarrow {{F_A}} = – \overrightarrow {{F_B}} \)
\(\overrightarrow {{F_A}} + \overrightarrow {{F_B}} = 0\)

It is to be noted that the action and reaction pair does not act on the same body but acts on two different bodies. So, action and reaction through equal in magnitude, never cancel out each other.

Now think of the case of pushing the wall of your room. In spite of pushing the wall of the room with all your might, the wall still stays at its position. You may even wonder whether is this a violation of Newton’s Third Law of Motion. No, you are surely exerting action on the wall, and the wall is also exerting reaction back on you. But the action and the reaction are not cancelling out each other. It is the force from the ground on the wall (The wall is indeed fixed to the ground, right?) that is balancing the force you are exerting on the wall.

Newton’s Third Law of Motion Statement

Newton’s Third Law of Motion states that:

“To every action, there is an equal and opposite reaction.”

That means the forces of action and reaction are always equal and opposite. These forces act on two different objects and never cancel each other. Each force produces its own effect based on the mass of the object.

Some Examples of Newton’s Third Law of Motion

Newton’s Third Law of Motion can be seen in many of our daily activities. Some of the examples are as under:

1. While we walk, we push the ground backwards, and the ground pushes us forward thus, enabling us to walk forward. Here our push on the ground in the backward direction is the action, and the push of the ground on us in the forward direction is the reaction.

2. Let us take the case of a book lying on the table. The book is exerting its weight in the downward direction on the table. So, the book is applying action on the table. In return, the table is exerting a normal reaction force in the upward direction on the book, which is the reaction applied by the table on the book. This is as shown in the below figure:

3. In hot air balloons or in the balloons filled with air, the air gushing out of the balloon experiences a force in the backward direction due to the balloon (action). In return, the air exerts a force on the balloon in the forward direction (reaction), which makes the balloon move in the forward direction. This can be seen in the below picture:

Applications of Newton’s Third Law of Motion

Newton’s Third Law of Motion finds many applications in our day-to-day life. Some of them are as under:

1. When the bullet is fired from a gun, the bullet going forward at high speed will exert a lot of force on the person holding the gun. The gun exerts action on the bullet in the forward direction, and in return, the bullet exerts reaction in the backward direction on the gun. Due to this reaction, the gun recoils and moves backwards through a small distance. The person holding the gun needs sufficient training to handle this backward movement of the gun. Thus, the concept of Newton’s Third Law of Motion helps the person understand this backward movement of the gun while firing.
2. The powerful stream of water coming out of the hosepipe of a fire engine to extinguish fire exerts a force on the hosepipe. Here, the water gushes out at high speed from the hosepipe in the forward direction, which is the action; its reaction in the backward direction is exerted on the hosepipe. So the hosepipe moves backwards when water moves in the forward direction. To hold the hosepipe at the proper position, the fireman needs adequate training. The concept of Newton’s Third Law of Motion helps the fireman understand the reason behind the backward movement of the hosepipe when water moves from it in the forward direction.
3. For the upward movement of the rocket, the fuel is burnt, which creates hot and highly compressed gases that are made to escape through a nozzle in the downward direction. This is the action exerted by the rocket on the ground. As a reaction, the ground pushes it upwards, and the rocket moves upwards. Here to use the reaction for the upward movement of the rocket action is routed in the downward direction in accordance with Newton’s Third Law of Motion.

Summary

To sum up, we can say that Newton’s Third Law of motion is the fundamental law that describes the very nature of force and helps us understand the fact that “forces always occur in pairs”. It is the fundamental law that lets us understand how forces act between two objects.

FAQs on Newton’s Third Law of Motion

The frequently asked questions based on Newton’s Third Law of Motion are as follows:

Q.1. What is the action and reaction pair in the case of a ball lying on the floor?
Ans: In the case of a ball lying on the floor, the weight of the ball is the action that is in the downward direction, and the normal reaction force of the floor on the ball is the reaction that is in the upward direction.

Q.2. What is the statement of Newton’s Third Law of Motion?
Ans: Newton’s Third Law of Motion states that to every action, there is an equal and opposite reaction.

Q.3. Does action and reaction act on the same body?
Ans: Action and reaction do not act on the same body. They act on two different bodies.

Q.4. Do action and reaction cancel each other?
Ans: As action and reaction act on two different bodies, they never cancel each other.

Q.5. Do action and reaction produce the same effect?
Ans: Action and reaction act on two different bodies, so the effects they produce depend on the mass of the bodies. If the bodies are of the same mass, then they may produce the same effect; else, the effects produced will be different.

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