18. NCERT line-by-line

1. TITLE

NCERT says 'Action and reaction forces act on different bodies, not on the same body' — Here's how NEET tests it

2. THE NCERT STATEMENT

NCERT EXACT LINE:
"Action and reaction forces act on different bodies, not on the same body. Consider a pair of bodies A and B... Thus if we are considering the motion of any one body (A or B), only one of the two forces is relevant. It is an error to add up the two forces and claim that the net force is zero."
— NCERT Class 11, Chapter 4, Page 56/57

3. WHY THIS LINE MATTERS

  • How many times has NEET tested this? Directly and indirectly, this fundamental concept appears almost every year, particularly when testing free-body diagrams, equilibrium, and conceptual assertion-reason questions.
  • What makes this statement tricky? Because Newton's Third Law famously states forces are "equal and opposite," students intuitively assume these forces should cancel each other out to produce a net force of zero.
  • What do students typically misremember? Students commonly misidentify two balancing forces acting on the same body (like the downward force of gravity, $W$, and the upward normal reaction, $R$, on a resting book) as an action-reaction pair.

4. MCQ (Directly from this NCERT line)

According to NCERT, Newton's Third Law of motion is characterized by which of the following?
A) Action and reaction forces act on different bodies, not on the same body.
B) Action and reaction forces act on the same body, not on different bodies.
C) Action and reaction forces occur sequentially (action causes reaction).
D) Action and reaction forces act on different bodies and can be added up to claim the net force is zero.

5. ANSWER + LINE BREAKDOWN

Correct Answer: A

Break down the NCERT line:
- "Action and reaction forces" means the simultaneous, mutual interaction between any pair of interacting bodies.
- "act on different bodies" means that if Body A exerts a force on Body B, the reaction is the equal and opposite force Body B simultaneously exerts back onto Body A.
- The key word is "relevant" because when analyzing the motion of a single body (like drawing a free-body diagram for Body A), you only consider the force acting on that specific body, meaning it is physically incorrect to add action and reaction pairs together to find a net force.

6. HOW NEET TWISTS THIS

Show 2-3 ways NEET can ask the same fact:
- As an assertion-reason:
Assertion: A book resting on a table is in equilibrium because the weight of the book and the normal reaction form an action-reaction pair.
Reason: Action and reaction forces act on the same body to keep it at rest.
(Answer: Both Assertion and Reason are False).
- As a direct conceptual question: "Why do action and reaction forces not cancel each other out?" (Answer: Because they act on different bodies).
- As an except question: "All of the following are true regarding Newton's Third Law EXCEPT..." (Incorrect option: They can be added together to find a net force of zero on a single particle).

7. MEMORY LOCK

MEMORIZE:
Action and reaction: Two bodies, two forces, zero cancellation!
Mnemonic: Different bodies = Don't cancel.

8. RELATED NCERT LINES

Also remember these from the same page:
- "There is no cause-effect relation implied in the third law. The force on A by B and the force on B by A act at the same instant."
- "However, if you are considering the system of two bodies as a whole, $F_{AB}$ and $F_{BA}$ are internal forces of the system (A + B). They add up to give a null force."

9. NCERT REFERENCE

NCERT Class 11, Chapter 4, Page 56, Paragraph 5

10. CTA

All important NCERT lines from this chapter → Laws of Motion Guide