NEET Physics β Newton's Laws of Motion
Newton's Third Law of Motion β Complete Notes, Revision, Important Questions & Downloads
Newton's Third Law states that to every action there is always an equal and opposite reaction: F_AB = βF_BA. NEET tests this topic through assertion-reason questions on the law's statement, identification-type questions ('Which of the following is a correct action-reaction pair?'), and applied problems distinguishing action-reaction pairs from two forces acting on the same body. Key NEET traps: action and reaction act on DIFFERENT bodies (so they never cancel each other), and forces in nature always occur in pairs β a single isolated force does not exist. The law applies to all types of forces: contact (normal, friction, tension) and non-contact (gravitational, electric).
Theory + ApplicationNewton's Laws Ch.4F_AB = βF_BA
Expected QuestionsQ
1β2
Newton's Third Law directly appears in assertion-reason format and application problems. Often paired with First Law comparisons. Appears in nearly every NEET paper cycle over 6 years.
Time Requiredβ±
60 min
20 min for law statement, action-reaction pair identification, and common misconceptions. 20 min for application examples (rocket, swimming, guns). 20 min for NEET-style assertion-reason and MCQ practice.
Difficultyβ‘
Easy-Medium
The statement is simple. The NEET difficulty comes from identifying correct action-reaction pairs (they must act on different bodies) and distinguishing them from balanced forces (which act on the same body). Assertion-reason format adds another layer β the reason must correctly explain the assertion.
NRI USA Curriculum GapUS
Low
AP Physics 1 fully covers Newton's Third Law, action-reaction pairs, and applications like rocket propulsion. The NEET-specific additions: emphasis on the formal law statement (verbatim), and assertion-reason question format testing the distinction between action-reaction pairs vs balanced forces on the same body.
7Subtopics
8+Practice Questions
4Free Downloads
60 minPrep Time
NEET Weightage β Newton's Third Law of Motion
Newton's Laws of Motion (Chapter 4)| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 1 | 4 | |
| 2023 | 1 | 4 | |
| 2022 | 1 | 4 | |
| 2021 | 0 | 0 | |
| 2020 | 1 | 4 | |
| 2019 | 1 | 4 | |
| 6-Year Total (2019β2024) | 2β4 | Β | 8β16 |
Statement: 'To every action, there is always an equal (in magnitude) and opposite (in direction) reaction.' Mathematically: F_AB = βF_BA, where F_AB is the force exerted by body A on body B (action), and F_BA is the force exerted by body B on body A (reaction).
Key property: Forces in nature always occur in pairs. A single isolated force is not possible. Action and reaction are always equal in magnitude, opposite in direction, collinear (along the same line of action), and SIMULTANEOUS (both exist at the same time β one does not cause the other in a time sequence).
Critical distinction: Action-reaction pairs act on DIFFERENT bodies. Two forces on the same body may sum to zero (balanced forces), but that is NOT Newton's Third Law β that is Newton's FIRST Law. Third Law pairs: ground pushes foot up (while foot pushes ground down), Earth pulls apple down (while apple pulls Earth up).
Key property: Forces in nature always occur in pairs. A single isolated force is not possible. Action and reaction are always equal in magnitude, opposite in direction, collinear (along the same line of action), and SIMULTANEOUS (both exist at the same time β one does not cause the other in a time sequence).
Critical distinction: Action-reaction pairs act on DIFFERENT bodies. Two forces on the same body may sum to zero (balanced forces), but that is NOT Newton's Third Law β that is Newton's FIRST Law. Third Law pairs: ground pushes foot up (while foot pushes ground down), Earth pulls apple down (while apple pulls Earth up).
0.8
Avg Questions / Year
20
Total Marks (6 yrs)
Direct
Pattern
Easy-Medium
Difficulty
How to Prepare Newton's Third Law for NEET
1
Memorise the exact law statement The NEET-standard statement: 'To every action, there is always an equal (in magnitude) and opposite (in direction) reaction.' Know the mathematical form: F_AB = βF_BA. Memorise both β assertion-reason questions quote the statement verbatim and test its every clause: equal magnitude, opposite direction, simultaneity, different bodies.
2
Practice action-reaction pair identification For every example, identify: (a) which body exerts the action and on which body, (b) which body exerts the reaction and on which body. They are always on different bodies and always equal + opposite. Common NEET examples: Book on table (book's weight = Earth pulling book; table's normal = table pushing book β these are NOT an action-reaction pair, because they act on the same body: the book).
3
Know the applications: swimming, rockets, guns Swimming: person pushes water backward (action) β water pushes person forward (reaction). Rocket: exhaust gases pushed backward (action) β rocket pushed forward (reaction). Gun: bullet pushed forward (action) β gun recoils backward (reaction). Each is a Third Law pair on DIFFERENT bodies (person/water, rocket/gases, bullet/gun). These are standard NEET scenario questions.
Study Materials β Newton's Third Law of Motion
PDF Β· Cheat Sheet Β· MCQ Set Β· PYQFull Notes
Law statement and formula. Properties of action-reaction pairs. Distinction from balanced forces. Examples: swimming, rocket, gun, horse-cart paradox. Worked assertion-reason problems.
Download NotesFormula Sheet
F_AB = βF_BA; |F_action| = |F_reaction|; action-reaction pair checklist: different bodies, equal magnitude, opposite direction, collinear, simultaneous.
Download SheetMCQ Practice
15 questions: law statement assertion-reason, action-reaction pair identification, horse-cart paradox, rocket propulsion concept, swimming direction explanation, action on different bodies.
Download MCQsPYQ
Year-tagged NEET questions on Newton's Third Law β statement type, identification of action-reaction pairs, and applied scenarios with full solutions.
Download PYQsSubtopics in Newton's Third Law of Motion
2-Column TableRapid Revision β Newton's Third Law of Motion
Concept β Trap β Example1) Law Statement and Properties
CoreTo every action, there is always an equal (in magnitude) and opposite (in direction) reaction. F_AB = βF_BA. Properties of action-reaction pairs: (1) Equal in magnitude. (2) Opposite in direction. (3) Collinear (act along the same line). (4) Simultaneous β both exist at the same instant; neither causes the other. (5) Act on DIFFERENT bodies.
- Forces in nature always occur in pairs. A single isolated force is not possible.
- Critical NEET distinction: A book on a table has two forces acting ON IT β weight (Earth pulls book) and normal (table pushes book). These are NOT an action-reaction pair because they act on the SAME body (the book). They happen to be equal and opposite (Newton's First Law) but that is coincidental (if the book were accelerating, they would NOT be equal). Third Law pairs: (A) Earth pulls book down + book pulls Earth up. (B) Table pushes book up + book pushes table down.
- Third Law pairs never appear in the free-body diagram of a single body β because by definition they act on two different bodies. If drawing an FBD and you see both members of a Third Law pair, you have drawn the FBD wrong.
Example (NEET-style)Gun firing a bullet: bullet experiences force F forward (action = gun pushes bullet). Gun experiences force F backward (reaction = bullet pushes gun). Both forces are simultaneous, equal in magnitude, opposite in direction. If bullet mass m = 0.01 kg, gun mass M = 1 kg, F = 500 N (time of firing Ξt = 0.002 s): bullet gains v = FΞt/m = 500Γ0.002/0.01 = 100 m/s forward. Gun gains V = FΞt/M = 500Γ0.002/1 = 1 m/s backward (recoil). Same force, different accelerations because masses differ.
2) The Horse-Cart Paradox
NEET-TrapIf the horse pulls the cart forward with force F, by Third Law the cart pulls the horse backward with EQUAL force F. So why does the system move? Because Newton's Third Law pairs act on DIFFERENT bodies: the horse-cart SYSTEM is subject to NET external forces only. The horse's hooves push the ground backward; the ground pushes horse+cart forward (friction). This forward ground-friction is the only external force on the system β it is NOT cancelled by the cart pulling the horse (which is an INTERNAL force within the system).
- Key insight: Third Law pairs are between DIFFERENT bodies. When analysing the entire system (horse+cart), internal forces cancel but external forces (ground friction) do not.
- If ground friction on horse's hoofs > friction of road on cart wheels (rolling friction), net external force > 0 β system accelerates forward. The horse-cart paradox is resolved by correctly identifying external vs internal forces.
- NEET question pattern: 'Is the horse-cart paradox a real contradiction of Newton's Third Law?' β It is NOT a contradiction. The Law is always valid; the error is mistakenly placing both Third Law forces on the same body in the analysis.
Example (NEET-style)Horse pulls cart with F = 200 N (forward on cart). Cart pulls horse backward with 200 N. Rolling friction on cart's wheels from road = 50 N backward. Ground friction on horse's hooves = 250 N forward. For the CART alone: Ξ£F = 200β50 = 150 N forward β cart accelerates. For the HORSE alone: Ξ£F = 250β200 = 50 N forward β horse accelerates. For the SYSTEM (horse+cart): Ξ£F_external = 250 (ground on horse) β 50 (road on cart) = 200 N forward β system accelerates. Internal forces (horse on cart, cart on horse) cancel. Consistent with Newton's Third Law.
3) Applications: Swimming, Rockets, Guns
ApplicationsEvery Third Law application involves identifying two different bodies and the mutual force pair. Swimming: swimmer pushes water backward (action) β water pushes swimmer forward (reaction). Rocket: exhaust gas pushed backward by engine (action) β rocket pushed forward (reaction). Gun: gun pushes bullet forward (action) β bullet pushes gun backward (reaction = recoil).
- In all these, motion occurs because the reaction force acts on the object we care about (swimmer, rocket, gun). The action ON the other body (water, gas, bullet) causes the reaction ON the primary.
- Rocket/Gun computation: momentum conservation (equivalent to Newton's Third Law integrated over time). At firing: m_bullet Γ v_bullet = M_gun Γ V_recoil (magnitudes). The lighter the gun, the larger the recoil velocity.
- NEET scenario: 'A man in a boat throws a heavy ball overboard. What happens to the boat?' β Ball pushed backward into water (action); boat+man pushed forward (reaction). The boat is NOT swimming to recover the ball; it moves opposite the direction the ball was thrown.
Example (NEET-style)Rocket propulsion: at time t, rocket (mass M) + gas ejects mass Ξm at velocity v_exhaust backward relative to rocket. By Third Law/momentum conservation: force on rocket = v_exhaust Γ (Ξm/Ξt) = thrust. If Ξm/Ξt = 50 kg/s and v_exhaust = 1000 m/s, thrust = 50,000 N forward on the rocket β regardless of the rocket's mass. As fuel burns, M decreases, so acceleration a = F/M increases over time. This is why the final stages of a rocket travel faster than early stages.
US Curriculum Gaps β Newton's Third Law of Motion
Topics in this section are tested in NEET but organised differently in standard US physics courses.Action-Reaction on Same vs Different Bodies (AP Physics 1 Gap)
AP Physics 1 explicitly teaches that action-reaction pairs act on different bodies. However, NEET assertion-reason questions specifically test whether students can distinguish between Third Law pairs (two forces on two different bodies) and balanced forces (two equal and opposite forces on the SAME body). The horse-cart paradox resolution β distinguishing internal and external forces when analysing multi-body systems β is tested at a depth in NEET that is less emphasised in AP Physics 1. Missing this distinction is the single most common NEET error on Newton's Third Law.
- AP Physics 1: Third Law taught conceptually, action-reaction pairs identified correctly
- NEET: assertion-reason on whether two forces on the SAME body constitute a Third Law pair (they do not)
- Horse-cart paradox: NEET standard assert-reason question type β less emphasised in AP curriculum
Forces in Nature Always Occur in Pairs (AP Physics 1 Gap)
The NCERT/NEET curriculum formalises the statement 'Forces in nature always occur in pairs; a single isolated force is not possible' as a direct consequence and corollary of Newton's Third Law. AP Physics 1 teaches the same concept but does not present it as a standalone corollary to be memorised verbatim. NEET frequently tests this as an assertion: 'Can a single isolated force exist?' (Answer: No, by Newton's Third Law). This statement-type question is a pure recall item that US-curriculum students may find unfamiliar as a testable claim.
- NCERT corollary: 'Forces in nature always occur in pairs. A single isolated force is not possible.'
- NEET: tests this as an assertion β true or false β and asks students to justify it using Third Law
- AP Physics 1: concept is taught but not stated as a named corollary to be memorised
NEET-Style Practice Questions β Newton's Third Law of Motion
4 Questions1According to Newton's Third Law, when a horse pulls a cart forward, which of the following is the correct action-reaction pair?Action-Reaction Identification
Horse pulls cart forward; ground pushes horse forward
Horse pulls cart forward; cart pulls horse backward
Cart pulls horse backward; horse pushes ground backward
Ground pushes cart forward; cart pushes ground backward
The correct Third Law pair is: Horse pulls cart forward (action on cart by horse) and Cart pulls horse backward (reaction on horse by cart). They are equal in magnitude, opposite in direction, and act on DIFFERENT bodies (the horse and the cart). Option A mixes forces on different systems (no direct action-reaction relationship between horse-on-cart and ground-on-horse). Option D is also a valid Third Law pair (cart pushes ground, ground pushes cart) but is not the action-reaction pair for 'horse pulls cart'. The horse-cart system moves because ground friction on the horse's hooves exceeds rolling friction on the cart's wheels β the NET external force on the system is forward.
2A book rests on a table. The two forces acting on the book β its weight (W) and the normal reaction (N) from the table β are equal in magnitude and opposite in direction. Are W and N an action-reaction pair (Newton's Third Law pair)?Same Body Distinction
Yes, because they are equal and opposite
No, because they both act on the SAME body (the book)
Yes, because they act simultaneously
No, because they act on different bodies
W and N are NOT a Newton's Third Law pair. They both act on the SAME body β the book. Newton's Third Law pairs act on DIFFERENT bodies. The actual Third Law pairs are: (1) Earth pulls book downward (W) β Book pulls Earth upward (W) β on Earth. (2) Table pushes book upward (N) β Book pushes table downward (N) β on the table. The reason W = N here is Newton's First Law (the book is in equilibrium) β not Third Law. If the table were accelerating, W β N, but the actual Third Law pairs would still be equal.
3Which statement about Newton's Third Law is WRONG?Statement Analysis
Action and reaction are always equal in magnitude
Action and reaction act on two different objects
Action and reaction can produce different accelerations in objects of different masses
Action and reaction can cancel each other to produce equilibrium
D is WRONG. Action and reaction CANNOT cancel each other because they act on DIFFERENT bodies. Forces cancel only when they act on the SAME body. Options A, B, and C are all correct statements about Newton's Third Law. (C) is correct because equal forces on unequal masses produce different accelerations: a = F/m. A small bullet (m small) gets large acceleration from the same force that gives the heavy gun (M large) small acceleration. D is the most common NEET misconception question β students who forget that Third Law pairs act on different bodies incorrectly believe they cancel.
4A man of mass 60 kg is standing in a lift. The lift accelerates upward at 2 m/sΒ². The man pushes the floor of the lift downward. What is the reaction force (by Newton's Third Law) on the man from the floor? (g = 10 m/sΒ²)Third Law Numerical
600 N
720 N
480 N
60 N
For the man accelerating upward: Ξ£F = ma β N β mg = ma β N = m(g+a) = 60(10+2) = 60Γ12 = 720 N. This is the normal force the floor exerts on the man (upward). By Newton's Third Law, the man pushes the floor downward with exactly 720 N (the reaction). So the reaction on the man FROM the floor = 720 N upward. Note: this is NOT equal to the man's weight (600 N) because the lift is accelerating. The Third Law pair: floor pushes man up (720 N) β man pushes floor down (720 N). These are equal by the Third Law regardless of acceleration.
Practice Problems β Newton's Third Law of Motion
Click "Reveal Answer" after attempting1A swimmer pushes water backward with a force of 200 N. The water pushes the swimmer forward. If the swimmer's mass is 80 kg, what is their acceleration? How does Newton's Third Law explain swimming?
1.25 m/sΒ², Third Law: water pushes swimmer forward with equal force
2.5 m/sΒ², swimmer is lighter than water
0.8 m/sΒ², only partial reaction occurs
200 m/sΒ², direct force application
π Reveal Answer
By Newton's Third Law: swimmer pushes water backward (action) β water pushes swimmer forward with equal force = 200 N (reaction). Acceleration of swimmer: a = F/m = 200/80 = 2.5 m/sΒ². However, this ignores the drag force of water on the swimmer's body (which opposes motion). In reality, the swimmer's net acceleration is less. For NEET: the key principle is that the swimmer moves forward because of the reaction force from water β swimming is entirely powered by Newton's Third Law. Without a fluid medium to push against, swimming is impossible.
2Explain why it is harder to walk on a frictionless surface (like ice) using Newton's Third Law. What force actually propels you forward?
The foot pushes backward; ground push forward (friction) propels you β without friction, no propulsion
Your leg muscles push you forward directly
Air resistance drives you forward
Normal force from the ground propels you
π Reveal Answer
When walking, the foot pushes the ground backward (action). By Newton's Third Law, the ground pushes the foot forward with equal force β this is static friction. This reaction (forward friction from ground) is what propels you forward. On a frictionless surface (ice), the ground cannot exert a backward-opposing friction force on the foot (no static friction). Therefore the Third Law reaction is negligible β the foot slides without generating a significant forward force. The more slippery the surface, the smaller the friction reaction, and thus the smaller the forward propulsion. This is why you slip on ice: no friction β no forward reaction β no walking.
3A gun of mass 5 kg fires a bullet of mass 50 g at a speed of 400 m/s relative to the ground. Find the recoil velocity of the gun using Newton's Third Law and conservation of momentum.
4 m/s backward
2 m/s backward
8 m/s backward
40 m/s backward
π Reveal Answer
By Newton's Third Law: the gun exerts a forward force on the bullet; the bullet exerts an equal backward force on the gun. Using conservation of momentum (initial momentum = 0 since both at rest): m_bullet Γ v_bullet + M_gun Γ V_gun = 0. So V_gun = β(m_bullet/M_gun) Γ v_bullet = β(0.050/5) Γ 400 = β0.01 Γ 400 = β4 m/s. The gun recoils at 4 m/s backward. Note: the recoil speed (4 m/s) is much smaller than the bullet speed (400 m/s) because the gun is 100Γ heavier than the bullet. Same force (Third Law) but very different accelerations because masses differ by factor 100.
4Two astronauts A (mass 60 kg) and B (mass 80 kg) are floating in space. A pushes B with a force of 120 N for 0.5 seconds. What happens to each astronaut? Apply Newton's Third Law.
A and B both move in the same direction
B moves away at 0.75 m/s; A moves in opposite direction at 1 m/s
Only B moves because A exerts the force
Neither moves because they are in space
π Reveal Answer
By Newton's Third Law: A pushes B with 120 N (action on B); B pushes A with 120 N in the opposite direction (reaction on A). Force A β B = 120 N for 0.5 s. Impulse on B: J = 120 Γ 0.5 = 60 NΒ·s. B's velocity: v_B = J/m_B = 60/80 = 0.75 m/s (in the direction A pushed). Impulse on A: J = 120 Γ 0.5 = 60 NΒ·s (opposite direction). A's velocity: v_A = 60/60 = 1 m/s (backward β opposite to B). Both astronauts move: B moves away at 0.75 m/s, A pushes back at 1 m/s. Total momentum check: 60Γ(β1) + 80Γ(0.75) = β60 + 60 = 0 β (initial momentum was zero β conservation satisfied).
Physics β Newton's Laws of Motion Revision Checklist
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FAQ β Newton's Third Law of Motion
Notes Β· Downloads Β· Revision Β· Important QuestionsWhat is Newton's Third Law of Motion?
Newton's Third Law states: 'To every action, there is always an equal (in magnitude) and opposite (in direction) reaction.' When body A exerts a force on body B (action), body B simultaneously exerts a force on body A (reaction) that is equal in magnitude and opposite in direction. Mathematically: F_AB = βF_BA. The law applies to all types of forces β gravitational, normal, tension, friction, electric, magnetic β and is universally valid at all speeds (in classical mechanics).
Do action and reaction cancel each other?
No. Action and reaction NEVER cancel each other because they act on DIFFERENT bodies. Forces cancel only when they act on the SAME body. The action F_AB acts on body B; the reaction F_BA acts on body A. Since they are on different bodies, they affect different bodies' motions and cannot be added algebraically in a single body's equation of motion. The common misconception: 'If action = βreaction, their sum = 0, so nothing moves.' This is wrong because we are adding forces on different objects β an invalid operation.
Is a single isolated force possible?
No. Newton's Third Law guarantees that forces always occur in pairs. Whenever one body exerts a force on another, the second body always exerts an equal and opposite force back on the first β simultaneously. A single isolated force (with no reaction) would violate Newton's Third Law. This is a universal statement: every contact force, gravitational force, electric force, and magnetic force in nature is one half of a Third Law pair.
What is the difference between a Third Law pair and balanced forces?
Third Law pair: two forces on TWO DIFFERENT bodies, always equal and opposite by Newton's Third Law. Example: Earth pulls apple down; apple pulls Earth up. Balanced forces: two or more forces on THE SAME body whose vector sum = zero (so acceleration = 0 β Newton's First Law). Example: book on table β weight (from Earth) and normal (from table) both act on the book and are equal and opposite. The book's weight and normal force are NOT a Third Law pair β they act on the same body and are only equal if the book is in equilibrium.
How does Newton's Third Law explain rocket propulsion?
The rocket engine burns fuel (propellant) and expels exhaust gases at high velocity backward (action: engine pushes gas backward). By Newton's Third Law, the exhaust gas exerts an equal force forward on the rocket (reaction: gas pushes rocket forward = thrust). The rocket needs no medium to push against (unlike a swimmer or car) β it creates its own reaction mass (exhaust). This is why rockets work in the vacuum of space. Thrust = v_exhaust Γ (dm/dt), where dm/dt is the mass flow rate of the exhaust.
Why does a gun recoil when fired?
When the gun fires the bullet, the expanding gunpowder gases exert a large forward force on the bullet (action). By Newton's Third Law, the bullet (through the gases) exerts an equal backward force on the gun (reaction = recoil). Both forces are equal in magnitude. But the gun (mass M) is much heavier than the bullet (mass m), so recoil velocity V = mΓv/M is much smaller than bullet velocity v (but non-zero). Example: m = 10 g, M = 2 kg, v = 400 m/s β V = (0.01 Γ 400)/2 = 2 m/s backward.
How does Newton's Third Law apply to a person walking?
When walking, the foot pushes backward on the ground (action). The ground pushes forward on the foot (reaction = static friction). This forward reaction from the ground is the actual force that accelerates the person forward. Without friction (icy surface), the ground cannot push forward β the person slips. The muscles in the leg provide energy but do NOT directly push the person forward: they push the foot on the ground, and the ground's Third Law reaction provides the forward propulsion.
Assertion: A horse cannot pull a cart because the cart pulls the horse back with equal force. Is this a correct application of Newton's Third Law?
The assertion is FALSE β it is a misapplication (the horse-cart paradox). While the cart does pull the horse backward with an equal force (Third Law), these two forces act on DIFFERENT bodies and cannot be summed to conclude 'no motion'. The correct analysis: for the horse+cart SYSTEM, only EXTERNAL forces matter. The ground exerts a forward friction force on the horse's hooves. This external force is not cancelled by any Third Law pair within the system, so the system accelerates forward. The horse-cart paradox is resolved by distinguishing between internal forces (cancel within the system) and external forces (drive the system's acceleration).
Are Third Law action-reaction forces simultaneous?
Yes, in classical mechanics, action and reaction forces are strictly simultaneous. They come into existence at the same instant and disappear at the same instant. Neither force 'causes' the other in a temporal sequence β they are a mutual, simultaneous interaction. This is why the language 'reaction FOLLOWS action' is physically inaccurate β there is no time delay. In electromagnetism (at large distances), there is a light-speed propagation delay in field interactions, but at the NEET level (classical mechanics with direct contact or near-field forces), the simultaneity assumption is correct.
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