Laws of Motion Notes for NEET and JEE Main 2027: Key Points and PDF

Laws of Motion is Chapter 4 of NCERT Class 11 Physics and Unit 3 of both the NEET UG and JEE Main syllabi. These notes cover inertia, Newton’s three laws, momentum and impulse, conservation of momentum, equilibrium of concurrent forces, static, kinetic and rolling friction, and circular motion on level and banked roads, with key facts and a free PDF.

Laws of Motion Notes for NEET and JEE Main

Section-wise key points from NCERT Chapter 4, friction and circular-motion tables, high-yield facts and common mistakes for 2027 revision.

Laws of Motion at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025 and the JEE Main 2026 syllabus published by the National Testing Agency (NTA). Both stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.

Aristotle’s Fallacy and the Law of Inertia

Aristotle held that an external force is needed to keep a body moving. Galileo showed this is wrong: bodies stop because of friction, and without it uniform motion would continue without any force.

• Aristotle (384 B.C. to 322 B.C.): ‘An external force is required to keep a body in motion.’ This is the flawed Aristotelian law.
• Galileo’s inclined planes: objects moving down accelerate, moving up retard, so on a frictionless horizontal plane they should move with constant velocity.
• Double inclined plane: a ball released on one smooth plane climbs to the same height on the other; if the second plane is made horizontal, it would travel an infinite distance.
• The state of rest and the state of uniform linear motion are equivalent: in both, the net external force is zero.
• Inertia means resistance to change of the state of rest or uniform motion.
• NCERT box on ancient Indian ideas: vega, the tendency to move in a straight line in the Vaisesika theory, comes closest to inertia; Bhaskara (1150 A.D.) introduced ‘instantaneous motion’ (tatkaliki gati).

Newton’s First Law of Motion

Every body continues in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise. Equivalently: if the net external force on a body is zero, its acceleration is zero.

• A spaceship in interstellar space with rockets off has no net force, so it keeps moving with uniform velocity.
• A book at rest on a table: because it is observed at rest, the first law says the net force is zero, so the normal force R equals the weight W. Saying ‘W = R, so the book is at rest’ reverses the reasoning.
• A car is accelerated by an external force, the friction from the road; internal forces cannot accelerate it.
• Passengers are thrown backward when a bus starts suddenly and forward when it stops suddenly, because of inertia.
• An astronaut who leaves a spaceship accelerating at 100 m s⁻² has zero acceleration the instant he is outside (no nearby stars).

Newton’s Second Law, Momentum and Impulse

The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts: F = dp/dt = ma.

• Momentum p = mv is a vector. A fixed force for a fixed time gives the same change in momentum to bodies of different mass.
• With the proportionality constant k = 1, 1 newton = 1 kg m s⁻² is the force that gives a mass of 1 kg an acceleration of 1 m s⁻².
• F = 0 gives a = 0, so the second law is consistent with the first.
• It is a vector law: Fₓ = maₓ, F_y = ma_y, F_z = ma_z. A force changes only the component of velocity along it; a projectile’s horizontal velocity stays unchanged under vertical gravity.
• For a system, F is the total external force and a is the acceleration of the centre of mass; internal forces are excluded.
• It is a local relation: acceleration here and now depends on the force here and now. A stone dropped from an accelerating train has no horizontal acceleration after release.

Impulse

• Impulse = force × time duration = change in momentum. Its unit is N s.
• An impulsive force is a large force acting for a short time; Newtonian mechanics treats it like any other force.
• A seasoned cricketer draws his hands back to increase the stopping time, which reduces the force needed to stop the ball.
• NCERT Example 4.4: a 0.15 kg ball hit straight back at 12 m s⁻¹ receives an impulse of 3.6 N s towards the bowler.

Newton’s Third Law and Conservation of Momentum

To every action there is always an equal and opposite reaction. Forces always occur in pairs: the force on A by B is equal and opposite to the force on B by A, F_AB = −F_BA.

• Action and reaction are simply forces; there is no cause and effect between them, and they act at the same instant.
• They act on different bodies, so they never cancel for one body. For the system (A + B) they are internal forces and add to zero.
• A falling stone pulls the earth with an equal and opposite force; the earth’s acceleration is negligible because of its huge mass.
• NCERT Example 4.5: when balls bounce off a rigid wall, the force on the wall is normal to the wall in both head-on and oblique cases.

Conservation of Momentum

• The total momentum of an isolated system of interacting particles is conserved.
• Gun and bullet: initial momentum is zero, so p_bullet + p_gun = 0 and the gun recoils.
• Internal forces change the momenta of individual particles, but the changes cancel in pairs.
• In a collision, total momentum before equals total momentum after, even when the forces during impact are complicated.

Equilibrium of a Particle and Common Forces

A particle is in equilibrium when the net external force on it is zero. For forces F₁, F₂ and F₃ acting on it, F₁ + F₂ + F₃ = 0, and the three forces can be represented by the sides of a triangle with arrows taken in the same sense.

• Equilibrium means each component of the net force is zero: ΣFₓ = 0, ΣF_y = 0, ΣF_z = 0.
• Gravity acts at a distance; the other common forces of mechanics are contact forces.
• The contact force between surfaces has a component normal to the surfaces (normal reaction) and a component parallel to them (friction).
• Buoyant force, viscous force and air resistance are contact forces from fluids.
• Spring force F = −kx, where k is the force constant; the minus sign shows it opposes the displacement.
• Tension is the restoring force in a string; it is taken as constant throughout a string of negligible mass.
• All contact forces of mechanics arise, at the microscopic level, from electrical forces between charged constituents.

Friction: Static, Kinetic and Rolling

Friction is the component of the contact force parallel to the surfaces, and it opposes relative motion (impending or actual) between them, not motion itself.

• The laws of friction are empirical and only approximately true, not fundamental laws.
• Angle of repose: a block on an incline just begins to slide when tan θ_max = μ_s, independent of mass. NCERT: θ_max = 15° gives μ_s = 0.27.
• A box on the floor of an accelerating train is accelerated by static friction: a_max = μ_s g (1.5 m s⁻² for μ_s = 0.15).
• Friction is reduced by lubricants, ball bearings and an air cushion; it is needed for walking, for cars to move and for brakes.

Circular Motion: Level and Banked Roads

A body moving in a circle of radius R at speed v needs a centripetal force f_c = mv²/R directed towards the centre.

• Stone on a string: centripetal force from tension. Planet around the sun: from gravity. Car on a level road: from static friction.

• NCERT Example 4.10: a cyclist at 18 km/h (5 m s⁻¹) on a turn of radius 3 m with μ_s = 0.1 slips, because v² = 25 m² s⁻² exceeds μ_s R g = 2.94 m² s⁻².
• NCERT Example 4.11: track of radius 300 m banked at 15° with μ_s = 0.2 gives an optimum speed of 28.1 m s⁻¹ and a maximum speed of about 38.1 m s⁻¹.

Solving Problems in Mechanics

NCERT’s method for any laws-of-motion problem:

• Draw a diagram showing the parts of the system and the links between them.
• Choose one convenient part of the system and draw its free-body diagram with all external forces on it (not the forces it exerts on others).
• Include known forces and leave unknowns as symbols; apply F = ma to that part.
• Use Newton’s third law to relate forces between parts, and repeat for other parts if needed.

Most-asked Facts for NEET and JEE Main: Laws of Motion

Revise these one-line facts from the chapter just before the exam.

• Galileo’s law of inertia was Newton’s starting point for the first law.
• Zero net force means zero acceleration, not zero velocity.
• F = dp/dt; for constant mass, F = ma; 1 N = 1 kg m s⁻².
• Impulse = F × Δt = Δp, measured in N s.
• Action and reaction act on different bodies and at the same instant.
• Total momentum of an isolated system is conserved; a gun recoils because p_gun = −p_bullet.
• Three concurrent forces in equilibrium form a closed triangle.
• Limiting friction (f_s)max = μ_s N; kinetic friction f_k = μ_k N with μ_k < μ_s.
• Friction does not depend on the area of contact.
• Angle of repose: tan θ_max = μ_s.
• Rolling friction is 2 to 3 orders of magnitude smaller than sliding friction.
• Level road: v_max = √(μ_s R g). Banked road, no friction: v₀ = √(Rg tan θ).

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Thinking a force is needed to keep a body moving uniformly. Only a net force of zero is needed; applied force just balances friction.
• Cancelling action and reaction for one body. They act on different bodies.
• Writing f_s = μ_s N for static friction in every case. Static friction only equals μ_s N at the limit; below that it equals the applied force.
• Saying friction always opposes motion. It opposes relative motion; static friction actually drives a box forward in an accelerating train.
• Adding ‘centripetal force’ as an extra force in a free-body diagram. It is supplied by tension, friction, gravity or the normal reaction.
• Assuming the stone dropped from an accelerating train keeps the train’s horizontal acceleration. After release, it has none.
• Using v_max = √(μ_s R g) for a banked road. Banked roads use the formula with tan θ.

Laws of Motion Notes PDF and Related Pages

Download the AglaSem notes for Laws of Motion (Newton’s laws of motion and friction key notes and important questions with answers) free: View PDF

• All chapters: Physics notes for NEET and JEE Main
• Full topic lists: NEET syllabus and JEE Main syllabus
• Practice: NEET question papers and JEE Main question papers
• Source text: NCERT Chapter 4 (NCERT, Class 11 Physics Part I)

Physics Notes for NEET and JEE Main: All Chapters

• Units and Measurement
• Motion in a Straight Line
• Motion in a Plane
• Laws of Motion
• Work, Energy and Power
• Systems of Particles and Rotational Motion
• Gravitation
• Mechanical Properties of Solids
• Mechanical Properties of Fluids
• Thermodynamics
• Kinetic Theory of Gases
• Oscillations
• Waves
• Thermal Properties of Matter
• Electric Charges and Fields
• Electrostatic Potential and Capacitance
• Current Electricity
• Moving Charges and Magnetism
• Magnetism and Matter
• Electromagnetic Induction
• Electromagnetic Waves
• Ray Optics and Optical Instruments
• Wave Optics
• Dual Nature of Radiation and Matter
• Atoms
• Nuclei
• Semiconductor Electronics

All chapters with key points and notes PDFs are on the Physics notes for NEET and JEE Main page.

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