Newton’s Third Law of Motion

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Newton’s Third Law of Motion is states that to every action, there is an equal (in magnitude) and opposite (indirection) reaction. If a body A exerts a force F on another body B then B exert a force – F on A.
The forces act along the same line. Action and reaction never cancel each other i.e., they can not balance each other as they act on different bodies. The force of action and reaction may appear due to actual physical contact of the two bodies or even from the distance.   
“For every action, there is an equal and opposite reaction.”
  • This statement means that forces always occur in pairs, where one object exerts a force on a second object, and the second object simultaneously exerts a force of equal magnitude but in the opposite direction on the first object. These paired forces are known as action-reaction forces.
  • This law is applicable whether the bodies are at rest or in the motion.
  • Single isolated force is not possible, force is always possible in action-reaction pair form.

To know better about Newton’s Third Law of Motion

To know better about Newton’s Third Law, we have to break it into few components:

1.Action and Reaction Are Forces:

    • The “action” is the force exerted by the first object on the second.
    • The “reaction” is the force exerted by the second object back on the first.
      2. Equal in Magnitude:
      • The action and reaction forces have the same strength, meaning their magnitudes are identical.
3. Opposite in Direction:
    • These forces act in exactly opposite directions.
      4. Act on Different Objects:
      • The action and reaction forces never act on the same object. This is why they do not cancel each other out.

Everyday Examples of Newton’s Third Law of motion

1.Walking: When we walk, our foot pushes backward against the ground (action), and the ground pushes forward against our foot (reaction), propelling us forward.
2. Rocket Propulsion: In a rocket, the engines expel gases downward (action), and the gases push the rocket upward (reaction), allowing it to launch into space.
3. Jumping: When we jump off the ground, we push downward on the ground (action), and the ground pushes us upward (reaction), allowing us to lift off.
4. Bird Flight: Birds push air downward with their wings (action), and the air pushes the birds upward (reaction), facilitate them to fly.
Newton's third law of motion-rocket upward
Rocket upward

Key Imminent

1.Interaction is Mutual: Newton’s Third Law of motion emphasises the mutual interaction between two objects. Neither the action nor the reaction exists in isolation; they are simultaneous and interdependent to each other.
2. Force Pairs in Different Systems: That the forces act on different objects helps clarify why they don’t cancel each other out. Example, in the case of a book resting on a table, the table exerts an upward reaction force equal to the book’s downward weight, allowing the book to stay at rest.
3.Application in Engineering: This law is primary in designing machines, vehicles, and structures. Like cars rely on the interaction between tires and road, where the tires push backward on the road, and the road pushes the car forward.
Newton's third law of motion-Vehicles
Vehicles

Misconceptions about Newton’s Third Law of Motion

1.Action and Reaction Are Simultaneous: Many people mistakenly believe that the action occurs first, followed by the reaction. In reality, both forces occur at the same time.
2. Force Pairs Do Not Cancel Out: Since action and reaction forces act on different objects, they do not cancel each other out. Example, when we sit on a chair, your weight acts downward on the chair, and the chair exerts an equal upward force on you.

Demonstration of Newton’s Third Law of Motion

A simple experiment to observe Newton’s Third Law of motion, using a balloon. Inflate a balloon and let it go without tying the end. The air escapes backward (action), and the balloon moves forward (reaction). This demonstrates how for deeper understanding of es act in opposite directions with equal magnitude.

Pseudo force

When the observer is in a non-inertial frame, then to apply Newton’s third law of motion we have to apply the imaginary force called pseudo force. Pseudo force is not the real force. It arises due to relative acceleration and not due to any interaction.
Newton’s third law of motion does not hold good for pseudo forces. The magnitude of pseudo force is the product of the mass of the body and the acceleration of the reference frame. The direction of pseudo force is opposite to the acceleration of the reference frame.   

Note

Newton’s Third Law of Motion is not just a principle for physics textbooks but a universal truth evident in countless daily activities. From walking and running to flying planes and launching rockets, this law underpins the mechanics of motion. By recognising and applying this principle, we come to know about how the objects interact and how forces shape the physical world.
Newton’s Third Law of Motion states: “For every action, there is an equal and opposite reaction.” It explains that forces between two interacting objects are always equal in magnitude and opposite in direction.
Action and reaction forces do not cancel each other out because they act on different objects. For example, when a book rests on a table, the book exerts a downward force on the table, and the table exerts an upward force on the book. Since these forces act on different objects, they cannot cancel.
A common example is walking. When we push backward on the ground with our foot (action), the ground pushes us forward (reaction), allowing us to move.
In a rocket, the engines expel gases downward (action), and the gases push the rocket upward (reaction), propelling it forward into space.
Yes, action and reaction forces always occur at the same time. One force cannot exist without the other; they are a pair force.
The principle helps engineers optimise tire-road interactions. When a car’s tires push backward on the road, the road pushes the car forward. This idea helps in designing efficient engines and traction systems.
Yes, it is. For example, when a swimmer pushes water backward with their hands (action), the water pushes the swimmer forward (reaction), allowing movement through the water.

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