Newton’s Third Law of Motion Physics Notes
Newton’s Third Law of Motion:
If an object ‘A exerts a force on object ‘B, then object B must exert a force of equal magnitude and opposite direction back on object A.
This law represents a certain symmetry in nature: forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as action-reaction, where the force exerted is the action and the force experienced as a consequence is the reaction.
According to Newton’s third law, “To every action, there is always an equal and opposite reaction”.
It must be remembered that action and reaction always act on different objects. The third law of motion indicates that when one object exerts a force on another object, the second object instantaneously exerts a force back on the object. These two forces are always equal in magnitude, but opposite in direction.
These forces act on different objects and so they do not cancel each other. Thus Newton’s third law of motion describes the relationship between the forces of interaction between two objects.
For example, when we placed a wooden block on the ground, this block exerts a force equal to its weight, W = mg acting downwards to the ground. This is the action force. The ground exerts an equal and opposite force N = mg on the block in upward direction. This is the reaction force.
Action weight of the body acting downwards
Illustrations of Newton’s Third Law:
Some of the examples of Newton’s third law of motion are given below:
1. A gun recoils when a bullet is fired form it: When a bullet is fired from a gun, the gun exerts a force on the bullet in the forward direction. This is the action force. The bullet also exerts an equal force on the gun in the backward direction. This is the reaction force. Due to the large mass of the gun it moves only a
little backward by giving a jerk at the shoulder of the gun man. The backward movement of the gun is called the recoil of the gun.
2. Walking: In order to walk, we press the ground in backward direction with our feet (action). In turns, the ground gives an equal and opposite reaction R, (figure (a)). The reaction R can be resolved into two components, one along the horizontal and other along the vertical. The component H = R cosθ along the horizontal, help us to move forward, while the vertical component, V = R sinθ opposes our weight, [figure (b)]
3. Swimming: While swimming, the swimmer pushes the water backward (action). The water pushes the swimmer forward (reaction) with the same force.
The swimmer pushes down and backwards against the water.
4. Flight of jet planes and rockets: Rockets move forward by expelling gas backward at high velocity. This means the rocket exerts a large backward force on the gas in the rocket combustion chamber, and the gas, therefore, exerts a large reaction force forward on the rocket. This reaction force is called thrust. It is a common misconception that rockets propel themselves by pushing on the ground or on the air behind them. They actually work better in a vacuum, where they can more readily expel the exhaust gases.
5. The flying of a bird: The wings of a bird force air a downward direction. In turn, the air gives equal and opposite reactions to the wings. The resultant reaction acts on the bird in an upward direction and makes the bird to fly upward.
Similarly, the Helicopter creates lift by pushing air down, thereby experiencing an upward reaction force. Birds and airplanes also fly by exerting force on air in a direction opposite to that of whatever force they need.
6. It is difficult to walk on sand or ice: As we press the sandy ground in the backward direction, the sarid gets pushed away and as a result, we get only a very small reaction from the sandy ground, making it difficult for us to walk. In case of ice, due to little friction between our feet and ice surface, there is hardly any forward reaction and hence we cannot walk on it.
7. Rebounding of a rubber ball: When a rubber is struck against a wall or floor, it exerts a force on wall (action). The ball rebounds with an equal force (reaction) exerted by the wall or floor on the ball.
Important Facts:
1. Forces of action and reaction act always on different bodies. Hence, they never cancel each other. Each force produces its own effect.
If we consider a pair of bodies A and B. then according to third law,
\(\vec{F}_{A B}=-\vec{F}_{B A}\)
i.e force on A by B= – force on B by A
2. If action and reaction forces were to act on the same body, their resultant would be zero.
3. It is wrong impression that action comes before reaction i. e., the action is the cause and reaction is the effect. The fact is that the two act at the same instant.
4. Newton’s third law is applicable for both kind of bodies i.e.. for bodies at rest or they are in motion.
5. Forces always occur in pairs.
6. The third law applies to all types of forces, e.g, gravitational, electric or magnetic forces etc.