Newton's Third Law — Revision Notes

⚡ 30-Second Revision

Statement: — For every action, there is an equal and opposite reaction.
Formula: — $vec{F}_{A \text{ on } B} = -vec{F}_{B \text{ on } A}$
Key Characteristics:

1. Equal magnitude. 2. Opposite direction. 3. Simultaneous. 4. Act on *different* bodies. 5. Are of the *same nature* (e.g., both gravitational, both normal).

Crucial Point: — Forces act on *different* bodies, so they *do not cancel* each other out.
Consequence: — Conservation of Linear Momentum in isolated systems.

2-Minute Revision

Newton's Third Law is fundamental: forces always come in pairs. If object A exerts a force (action) on object B, then object B simultaneously exerts an equal and opposite force (reaction) on object A.

These forces, $vec{F}_{AB}$ and $vec{F}_{BA}$, are always equal in magnitude, opposite in direction, and occur at the same instant. The most critical point for NEET is that action and reaction forces *always act on different bodies*.

This means they cannot cancel each other out to determine the net force on a single object. For example, when you push a wall, you exert force on the wall, and the wall exerts force on you. The wall's motion is affected by the force you exert on it, and your motion is affected by the force the wall exerts on you.

This law directly leads to the principle of conservation of linear momentum, crucial for problems involving collisions and explosions. Always distinguish action-reaction pairs from balanced forces, which act on the same object.

5-Minute Revision

Newton's Third Law, often stated as 'for every action, there is an equal and opposite reaction,' is a cornerstone of mechanics. It describes the fundamental nature of force as an interaction between two objects. When object A exerts a force $vec{F}_{AB}$ on object B, object B simultaneously exerts a force $vec{F}_{BA}$ on object A, such that $vec{F}_{AB} = -vec{F}_{BA}$.

Key Properties of Action-Reaction Pairs:

1
Equal Magnitude: — The strength of the action force is precisely equal to the strength of the reaction force.
2
Opposite Direction: — The forces point in exactly opposite directions.
3
Simultaneous: — They occur at the exact same instant; there's no delay.
4
Act on Different Bodies: — This is paramount. The action force acts on one body, and the reaction force acts on the *other* body. This means they cannot cancel each other out when considering the net force on a single object. For instance, if you push a cart, the force you exert on the cart makes the cart accelerate, while the force the cart exerts on you affects your motion.
5
Same Nature: — If the action is gravitational, the reaction is gravitational. If it's a normal force, the reaction is also a normal force.

Common Applications:

Walking: — Foot pushes Earth backward (action), Earth pushes foot forward (reaction).
Rocket Propulsion: — Rocket expels gas downward (action), gas pushes rocket upward (reaction).
Recoil of a Gun: — Gun pushes bullet forward (action), bullet pushes gun backward (reaction).

NEET Focus:

Distinguishing from Balanced Forces: — Balanced forces act on the *same* object and sum to zero (e.g., gravity and normal force on a book). Action-reaction pairs act on *different* objects.
Conservation of Momentum: — This law is a direct consequence of N3L. For an isolated system, the total momentum remains constant ($sum vec{p} = \text{constant}$). This is vital for collision and explosion problems.
Free Body Diagrams: — Correctly identifying action-reaction pairs helps in drawing accurate FBDs for individual components of a system, crucial for applying Newton's Second Law.

Example: A $10,\text{kg}$ block is pushed by a $20,\text{N}$ force on a frictionless surface. What is the force exerted by the block on the hand? The hand exerts $20,\text{N}$ on the block (action). By N3L, the block exerts $20,\text{N}$ on the hand (reaction), in the opposite direction. The block accelerates due to the $20,\text{N}$ force on *it*.

Prelims Revision Notes

Newton's Third Law is a cornerstone of mechanics, stating that forces always occur in pairs. For every action, there is an equal and opposite reaction. This means if object A exerts a force $vec{F}_{AB}$ on object B, then object B simultaneously exerts a force $vec{F}_{BA}$ on object A, such that $vec{F}_{AB} = -vec{F}_{BA}$.

Key Characteristics to Remember for NEET:

1
Magnitude: — Always equal. $|F_{action}| = |F_{reaction}|$.
2
Direction: — Always opposite. $vec{F}_{action}$ is opposite to $vec{F}_{reaction}$.
3
Simultaneity: — They occur at the same instant. No delay.
4
Acting on Different Bodies: — This is the most crucial point. The action force acts on one body, and the reaction force acts on the *other* body. Because they act on different bodies, they *cannot cancel each other out* to produce zero net force on a single body. This is a frequent trap in MCQs.
5
Nature of Forces: — They are always of the same type (e.g., both gravitational, both normal, both frictional).

Common Misconceptions & NEET Traps:

Cancellation: — Do not assume action-reaction pairs cancel. They affect the motion of *different* objects.
Balanced Forces vs. Action-Reaction: — Balanced forces act on the *same* object and sum to zero (e.g., normal force and gravity on a stationary book). Action-reaction pairs act on *different* objects.

Applications & Consequences:

Walking: — Foot pushes ground backward (friction), ground pushes foot forward (friction).
Rocket Propulsion: — Rocket expels gases (action), gases push rocket (reaction).
Recoil of a Gun: — Gun pushes bullet (action), bullet pushes gun (reaction).
Conservation of Linear Momentum: — This is a direct consequence of Newton's Third Law. In an isolated system, the total momentum ($sum vec{p}$) remains constant. This is vital for problems involving collisions and explosions ($m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2$).

Problem-Solving Strategy:

1
Identify the Interaction: — Clearly define the two objects interacting.
2
Identify Action Force: — State 'Object 1 exerts force on Object 2'.
3
Identify Reaction Force: — State 'Object 2 exerts force on Object 1'. Ensure it's equal in magnitude and opposite in direction.
4
Free Body Diagrams: — When drawing FBDs, only include forces *acting on* the specific body. The reaction forces will be on other bodies.

Mastering these points will help you correctly analyze force interactions and solve related problems in NEET.

Vyyuha Quick Recall

Always React On Different Bodies

Always React: Action and Reaction are always present.
On: Opposite in direction.
Different Bodies: Crucially, they act on different objects, so they don't cancel.