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CLASS 11 PHYSICS • LAWS OF MOTION

Newton's Third Law and Applications

Understand action-reaction pairs, recoil, walking, rocket propulsion, pulley systems and exam-level applications.

CBSENEETJEE MainJEE AdvancedIBIGCSEA-Level

Section 1

Newton's Third Law

Newton's Third Law states that for every action, there is an equal and opposite reaction. These forces are simultaneous interaction forces between two different bodies.

Action-reaction forces are equal in magnitude and opposite in direction, but they do not cancel each other because they act on different bodies. Internal forces cancel only when the whole system is considered.

Equal forcesOpposite directionsDifferent bodiesInteraction pairInternal vs external

Book on table

Table pushes book upward; book pushes table downward. These are on different bodies.

Hand pushing object

Hand pushes object forward and object pushes hand backward with equal force.

Magnet interaction

Each magnet exerts an equal and opposite force on the other magnet.

Section 2

Action-Reaction Pair

To identify a third-law pair, write force on A by B and force on B by A. The two forces must involve the same interaction and must act on different bodies.

Situation	Action	Reaction	Trap
Book on table	Book pushes table down	Table pushes book up	Weight and normal are not a third-law pair
Walking	Foot pushes ground backward	Ground pushes foot forward	Friction is helpful, not always opposing motion
Rocket	Rocket pushes gas backward	Gas pushes rocket forward	Rocket does not need air to push against
Pulley	String pulls block	Block pulls string	Tensions on one block are not action-reaction pair

NEET trapJEE Main trapJEE Advanced reasoningIB explanationIGCSE basicsA-Level vector pair

Section 3

Recoil

When a bullet is fired, the gun pushes the bullet forward and the bullet pushes the gun backward. For the bullet-gun system, external horizontal force is negligible during the short firing interval.

0 = m_bv_b + m_gv_g

Thus recoil velocity of gun is opposite to bullet velocity.

Advanced NumericalShow Answer

Question: A bullet of mass 0.01 kg leaves a 2 kg gun at 400 m/s. Find recoil speed.

Answer: 0 = 0.01 x 400 + 2v, so v = -2 m/s. Gun recoils at 2 m/s backward.

Section 4

Walking

Walking is possible because of static friction. The foot pushes the ground backward, and the ground pushes the foot forward. On a slippery surface, friction is too small to provide enough forward reaction.

Conceptual QuestionShow Answer

Question: Why is walking difficult on ice?

Answer: Ice provides very small static friction, so the ground cannot exert enough forward reaction force on the foot.

Section 5

Rocket Propulsion

A rocket ejects gases downward/backward at high speed. By Newton's Third Law, gases push the rocket upward/forward. Momentum of rocket plus exhaust gases is conserved approximately over a short interval.

thrust = rate of change of momentum of exhaust gases

Basic rocket equation idea: continuous mass ejection changes rocket momentum and produces thrust.

Rocket Numerical/ConceptShow Answer

Question: Why can a rocket move in space?

Answer: It pushes exhaust gases backward. The gases push the rocket forward. It does not need air outside.

Section 6

Pulley Systems

In pulley systems, tension is the pulling force transmitted by a string. The string pulls the block, and the block pulls the string with equal and opposite force. Fixed pulleys change direction of force, while movable pulleys can reduce effort.

Single pulley: one string tension acts on the block.
Double pulley: multiple string segments support load.
Fixed pulley: changes direction.
Movable pulley: gives mechanical advantage.

Section 7

Applications

Walking

Concept: Friction makes forward motion possible.

Action force: Foot pushes ground backward.

Reaction force: Ground pushes foot forward.

Swimming

Concept: Water interaction moves swimmer.

Action force: Swimmer pushes water backward.

Reaction force: Water pushes swimmer forward.

Flying birds

Concept: Wings interact with air.

Action force: Wings push air downward/backward.

Reaction force: Air pushes wings upward/forward.

Jet engine

Concept: Exhaust gases create thrust.

Action force: Engine pushes gases backward.

Reaction force: Gases push engine forward.

Rocket

Concept: Momentum conservation in rocket-gas system.

Action force: Rocket pushes gases downward/backward.

Reaction force: Gases push rocket upward/forward.

Gun recoil

Concept: Bullet and gun exert forces on each other.

Action force: Gun pushes bullet forward.

Reaction force: Bullet pushes gun backward.

Hose pipe recoil

Concept: Fast water jet gives recoil.

Action force: Hose pushes water forward.

Reaction force: Water pushes hose backward.

Balloon propulsion

Concept: Escaping air provides thrust.

Action force: Balloon pushes air backward.

Reaction force: Air pushes balloon forward.

Rowing boat

Concept: Oar-water interaction moves boat.

Action force: Oar pushes water backward.

Reaction force: Water pushes oar and boat forward.

Skating

Concept: Skaters push each other apart.

Action force: Skater A pushes skater B.

Reaction force: Skater B pushes skater A.

Section 8

High-Quality Numericals

CBSE 1: Wall push

Question: A student pushes a wall with 80 N. What force does wall exert on student?

Given: Action force = 80 N

Formula: F reaction = -F action

Calculation: Wall exerts 80 N opposite to push.

Final Answer: 80 N opposite

Exam Tip: Forces act on different bodies.

NEET 1: Gun recoil

Question: A 0.02 kg bullet leaves a 5 kg gun at 500 m/s. Find recoil speed.

Given: mb=0.02 kg, vb=500 m/s, mg=5 kg

Formula: 0 = m_bv_b + m_gv_g

Calculation: vg = -(0.02 x 500)/5 = -2 m/s

Final Answer: 2 m/s backward

Exam Tip: Use momentum conservation for gun-bullet system.

JEE Main 1: Skaters

Question: A 40 kg skater pushes a 60 kg skater. If 40 kg skater moves at 3 m/s, find other speed.

Given: m1=40, v1=3, m2=60

Formula: m₁v₁ + m₂v₂ = 0

Calculation: 120 + 60v2 = 0, v2 = -2 m/s

Final Answer: 2 m/s opposite

Exam Tip: Internal forces conserve total momentum.

JEE Advanced 1: Pulley pair

Question: Two equal masses hang over a light pulley. Find acceleration.

Given: m and m

Formula: Net force = 0

Calculation: Equal weights balance; acceleration is zero.

Final Answer: 0

Exam Tip: Tension acts on each mass, but action-reaction pairs are on different bodies.

IB 1: Walking

Question: Explain walking on a rough floor using third law.

Given: Friction present

Formula: Action-reaction pair

Calculation: Foot pushes floor backward, floor pushes foot forward.

Final Answer: Forward motion possible

Exam Tip: Static friction is essential.

IGCSE 1: Rocket

Question: Why does rocket move upward?

Given: Gases ejected downward

Formula: Action-reaction

Calculation: Rocket pushes gases down; gases push rocket up.

Final Answer: Upward thrust

Exam Tip: Rocket can work in space.

A-Level 1: Rifle recoil

Question: Compare recoil for heavier rifle.

Given: same bullet momentum, larger rifle mass

Formula: v = p/M

Calculation: Larger M gives smaller recoil speed.

Final Answer: Smaller recoil

Exam Tip: Momentum is conserved, not kinetic energy necessarily.

Section 9

NEET Question Bank

NEET Exam-style QuestionShow Answer

Question: NEET 1: In a question on third law, choose the correct statement. Options: (A) Action and reaction act on the same body. (B) Action and reaction are equal, opposite and act on different bodies. (C) Reaction is always smaller. (D) Action happens first and reaction later.