Quick Answer: Newton’s three laws of motion state that (1) an object stays at rest or in uniform motion unless an external force acts on it, (2) the net force on a body equals its mass times acceleration (F = ma, or force = rate of change of momentum), and (3) every action has an equal and opposite reaction acting on two different bodies. These laws form the backbone of mechanics and appear in nearly every NEET and JEE Physics paper.
- Why Laws of Motion Matter for NEET & JEE
- Newton’s First Law (Law of Inertia): Concept Explained
- PYQ-Style Applications of the First Law
- Newton’s Second Law: F = ma Demystified
- PYQ-Style Applications of the Second Law
- Newton’s Third Law: Action-Reaction Pairs
- PYQ-Style Applications of the Third Law
- Free Body Diagrams: The Exam Solver’s Tool
- Common Mistakes Aspirants Make in Laws of Motion
- Quick Revision Table: Formulas & Key Points
- Frequently Asked Questions
- Q: What is Newton’s first law of motion with an example?
- Q: Is F = ma enough for JEE questions on laws of motion?
- Q: Do action-reaction forces cancel out?
- Q:How many questions from laws of motion appear in NEET?
- Q: What is inertia and its types?
- Related reading
Why Laws of Motion Matter for NEET & JEE
Laws of motion is one of the highest-yield chapters in Class 11 mechanics. In NEET, typically one to two questions appear every year from this chapter (often blended with friction or circular motion). In JEE Main, questions from Newton’s laws, free body diagrams, pulleys, and inclined planes appear regularly — sometimes two to three questions in a single shift. JEE Advanced goes further, testing constraint relations, impulse-momentum, and system-of-particles reasoning.
Typical PYQ patterns include:
- Conceptual MCQs on inertia and action-reaction pairs (NEET favourite).
- Numericals on F = ma and impulse with momentum change (both NEET and JEE Main).
- Two-block and pulley problems solvable only through free body diagrams (JEE).
- Inclined-plane questions combining Newton’s second law with friction.
For official syllabus confirmation, always check the NEET information bulletin on nta.ac.in and the JEE Main page on jeemain.nta.nic.in. Reference-level treatment is available in NCERT Class 11 Physics, Chapter 4 (Laws of Motion) via ncert.nic.in.
Newton’s First Law (Law of Inertia): Concept Explained
Statement: A body remains at rest or continues to move with uniform velocity in a straight line unless compelled by an external net force to change that state.
The property behind this law is inertia — the natural resistance of a body to any change in its state of motion. Inertia is measured by mass: the more massive a body, the greater its inertia.
Types of inertia:
- Inertia of rest — a resting body stays at rest. Example: dust particles fall off a carpet when it is beaten with a stick; the carpet moves, the dust tends to stay behind.
- Inertia of motion — a moving body keeps moving. Example: a passenger jerks forward when a bus brakes suddenly; the lower body stops with the bus, the upper body continues forward.
- Inertia of direction — a body resists a change in direction. Example: sparks flying tangentially off a grinding wheel.
Another classic: when a bus starts suddenly, passengers fall backward — their bodies at rest tend to remain at rest while the bus moves forward.
PYQ-Style Applications of the First Law
MCQ 1: A passenger sitting in a bus moving at constant velocity throws a ball straight up. Where does the ball land?
Answer: Back in the thrower’s hands. Because of inertia of motion, the ball retains the bus’s horizontal velocity throughout its flight (first law — no horizontal force acts on it, ignoring air resistance).
MCQ 2: Which quantity measures the inertia of a body?
Answer: Mass. Weight varies with gravity (W = mg), but mass — and hence inertia — is the same everywhere.
MCQ 3 (Assertion–Reason): Assertion: A body can have zero net force and still be moving. Reason: Without net force, velocity remains constant.
Answer: Both are true, and the reason correctly explains the assertion — this is exactly the first law.
Newton’s Second Law: F = ma Demystified
Statement: The rate of change of momentum of a body is directly proportional to the net applied force and occurs in the direction of the force.
In equation form:
F = ma (from F = dp/dt, where p = mv)
Key points:
- The SI unit of force is the newton (N): 1 N = 1 kg·m/s², the force that gives a 1 kg mass an acceleration of 1 m/s².
- F in F = ma is the net (resultant) force, not any single force.
- Mass vs weight: mass (kg) is the quantity of matter and measure of inertia; weight (N) is the gravitational force on that mass (W = mg). Mass is scalar and invariant; weight is a force, vector, and location-dependent.
- Impulse = FΔt = Δp — the momentum form is heavily tested in JEE.
PYQ-Style Applications of the Second Law
Problem 1 (force): A 5 kg block on a frictionless surface accelerates at 2 m/s². Find the net force.
Solution: F = ma = 5 × 2 = 10 N.
Problem 2 (impulse): A 0.15 kg cricket ball moving at 20 m/s is caught and brought to rest in 0.1 s. Find the average force on the hands.
Solution: Δp = m(v − u) = 0.15 × (0 − 20) = −3 kg·m/s. F = Δp/Δt = −3/0.1 = −30 N (30 N opposing the ball’s motion).
Problem 3 (elevator): A 60 kg person stands in a lift accelerating upward at 2 m/s² (g = 10 m/s²). Find the normal reaction.
Solution: N − mg = ma → N = m(g + a) = 60 × 12 = 720 N. This “apparent weight” pattern appears repeatedly in NEET papers.
Problem 4 (two blocks): Blocks of 2 kg and 3 kg are connected on a frictionless floor and 10 N is applied to the 2 kg block. Find the acceleration and contact force.
Solution: a = 10/(2+3) = 2 m/s²; contact force on the 3 kg block = 3 × 2 = 6 N.
Newton’s Third Law: Action-Reaction Pairs
Statement: For every action, there is an equal and opposite reaction. Action and reaction:
- Are equal in magnitude, opposite in direction.
- Act on two different bodies — never on the same body.
- Act simultaneously and are the same type of force (both gravitational, both normal, etc.).
Daily-life examples:
- Walking: you push the ground backward; the ground pushes you forward.
- Rocket propulsion: hot gases are expelled downward at high speed; the rocket is pushed upward — no air is needed, which is why rockets work in space.
- Swimming: the swimmer pushes water backward; water pushes the swimmer forward.
- Gun recoil: the gun pushes the bullet forward; the bullet pushes the gun backward.
PYQ-Style Applications of the Third Law
Trap MCQ 1: If action and reaction are equal and opposite, why don’t they cancel out?
Answer: Because they act on two different bodies. Forces can cancel (balance) only when they act on the same body. When you push a wall, your force on the wall and the wall’s force on you are a pair — each body experiences only one of them.
Trap MCQ 2: A book rests on a table. Is the normal force from the table the “reaction” to the book’s weight?
Answer: No. The normal force and weight both act on the book — they balance but are not an action-reaction pair. The reaction to the book’s weight is the book pulling the Earth upward; the reaction to the normal force is the book pressing down on the table. This is a classic NEET trap.
MCQ 3: A rocket works in vacuum because of —
Answer: Newton’s third law (momentum conservation): expelled gases provide thrust regardless of any surrounding medium.
Free Body Diagrams: The Exam Solver’s Tool
A free body diagram (FBD) isolates one body and shows all external forces acting on it. Follow this method:
- Choose one body at a time (one block, one pulley, one person).
- Draw all forces acting on that body — weight (mg downward), normal reaction, tension, friction, applied forces.
- Choose axes. For inclined planes, take x along the incline and y perpendicular to it.
- Resolve forces into components along the axes. On a frictionless incline of angle θ: mg sin θ drives motion, N = mg cos θ.
- Apply ΣF = ma along each axis for each body.
- Solve the simultaneous equations, using constraints (e.g., same tension and same acceleration magnitude for a light, inextensible string over a frictionless pulley).
Worked example: A 4 kg block on a 30° frictionless incline (g = 10 m/s²). Acceleration = g sin 30° = 10 × 0.5 = 5 m/s²; N = mg cos 30° ≈ 34.6 N.
Common Mistakes Aspirants Make in Laws of Motion
- Sign errors: taking “up the incline” as positive in one equation and down in another. Fix your axis convention before writing ΣF = ma.
- Mass vs weight confusion: substituting kg (mass) where newtons (weight) are needed. Remember W = mg; on the Moon, mass stays the same, weight drops to about one-sixth.
- Treating any equal-opposite pair as action-reaction: only forces on different bodies qualify (see the book-on-table trap above).
- Using F = ma with a single force instead of the net force.
- Forgetting pseudo-forces in non-inertial (accelerating) frames — a JEE Advanced favourite.
- Ignoring the string constraint in pulley systems, assuming both masses have the same acceleration when pulleys move.
Quick Revision Table: Formulas & Key Points
| Concept | Formula / Fact | SI Unit / Memory Hook |
|---|---|---|
| First law | Body keeps its state unless net external force acts | “Law of inertia” — bus brake: jerk forward |
| Second law | F = ma = dp/dt | Newton (N) = kg·m/s² |
| Momentum | p = mv | kg·m/s |
| Impulse | J = FΔt = Δp | N·s; cricketer lowers hands to increase Δt, reduce F |
| Third law | Action = −Reaction, on different bodies | Rocket: gases down, rocket up |
| Weight | W = mg | Newton, not kg |
| Incline (frictionless) | a = g sin θ, N = mg cos θ | Resolve mg along/perpendicular to slope |
| Apparent weight in lift | N = m(g ± a) | + going up, − coming down |
Frequently Asked Questions
Q: What is Newton’s first law of motion with an example?
An object stays at rest or in uniform motion unless an external force acts on it. Example: a passenger jerks forward when a bus stops suddenly — the lower body stops with the bus, but the upper body continues moving due to inertia.
Q: Is F = ma enough for JEE questions on laws of motion?
No. JEE also tests the momentum form (F = dp/dt), impulse, friction, pseudo-forces in accelerating frames, and constraint relations — all of which must be handled through free body diagrams. F = ma is only the starting point.
Q: Do action-reaction forces cancel out?
No. They act on two different bodies, and only forces acting on the same body can cancel in equilibrium. Each body in the pair experiences only one of the two forces.
Q:How many questions from laws of motion appear in NEET?
Typically one to two questions from mechanics/laws of motion appear in NEET Physics each year, often combined with friction or circular motion. Verify with the latest official papers on nta.ac.in, as weightage can shift slightly.
Q: What is inertia and its types?
Inertia is a body’s resistance to any change in its state of motion, measured by its mass. Its three types are inertia of rest (dust off a beaten carpet), inertia of motion (passenger leaning forward when a bus brakes), and inertia of direction (sparks flying tangentially off a grinding wheel).
Related reading
- Vertical Motion and Free Fall: Gravity's Signature Move
- Motion Graphs: Reading a Journey Like a Sentence
Quick revision
- Conceptual MCQs on inertia and action-reaction pairs (NEET favourite).
- Numericals on F = ma and impulse with momentum change (both NEET and JEE Main).
- Two-block and pulley problems solvable only through free body diagrams (JEE).
- Inclined-plane questions combining Newton’s second law with friction.
- Inertia of rest: — a resting body stays at rest.
- Inertia of motion: — a moving body keeps moving.
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