Laws of Motion for NEET: Newton's Laws, Friction, Traps
The laws of motion are Newton's three rules that link force to change in motion: a body keeps its velocity unless a net external force acts, the net force equals the rate of change of momentum, and forces always come in equal and opposite pairs. In the NEET (UG) syllabus, Laws of Motion is Physics Unit 3, and it also covers impulse, conservation of momentum, friction and circular motion on roads. This guide takes the laws of motion for NEET phrase by phrase, works the NCERT examples with their real numbers, and names the traps that quietly cost marks.
What does Unit 3 of the NEET syllabus include?
Unit 3 includes exactly the phrases printed in the syllabus. The NMC syllabus for NEET (UG) 2026, issued by the Undergraduate Medical Education Board on 22 December 2025, gives the laws of motion four printed lines. Split into phrases, this is what each one expects you to do.
| Syllabus phrase (Unit 3) | What you should be able to do |
|---|---|
| Force and inertia, Newton's First law of motion | Decide from the state of motion whether the net force is zero |
| Momentum, Newton's Second Law of motion | Use F = dp/dt and F = ma, and resolve forces into components |
| Impulses | Find impulse as change in momentum, J = FΔt = Δp |
| Newton's Third Law of motion | Name action and reaction pairs and the bodies they act on |
| Law of conservation of linear momentum and its applications | Solve recoil, explosion and collision momentum problems |
| Equilibrium of concurrent forces | Make the vector sum of three or more forces zero |
| Static and Kinetic friction, laws of friction, rolling friction | Use fs ≤ µsN and fk = µkN, and know µk is less than µs |
| Dynamics of uniform circular motion: centripetal force | Find the real force that supplies mv²/R |
| Vehicle on a level circular road, vehicle on a banked road | Use v max = √(µsRg) and the banked road formulas |
Two neighbouring ideas sit in other units, which matters when you plan revision. The relations for uniformly accelerated motion (the equations of motion) and the frame of reference belong to Unit 2, Kinematics. Motion in a vertical circle is printed in Unit 4, Work, Energy and Power. And the words "pseudo force" and "inertial frame" appear nowhere in the 2026 syllabus or in the NCERT chapter, which solves every lift and train problem from the ground.
How many marks can laws of motion decide?
Laws of motion can decide more marks than its own questions, because almost every mechanics problem starts with a free body diagram and F = ma. The NEET (UG) 2026 information bulletin fixes Physics at 45 questions and 180 of the 720 marks, in a three hour paper. NTA does not publish how many questions come from each unit, so any precise chapter count you see online is somebody's estimate.
The marking makes accuracy worth more than attempts. A right answer is plus four, a wrong one minus one, a blank zero. Two wrong guesses cancel half of a correct answer, which is why our guide to negative marking in NEET is worth reading before your next mock.
Which NCERT chapter covers the laws of motion?
The laws of motion are Chapter 4 of NCERT Physics Class 11 Part I, reprint 2026-27. The chapter runs in eleven sections, and the order is worth copying for your own revision.
| NCERT section | What it gives you |
|---|---|
| 4.2 and 4.3 | Aristotle's fallacy and Galileo's law of inertia |
| 4.4 to 4.6 | Newton's first, second and third laws, with impulse |
| 4.7 | Conservation of momentum, gun and bullet, collisions |
| 4.8 | Equilibrium of a particle under concurrent forces |
| 4.9 | Common forces: contact forces, tension, spring force, friction |
| 4.10 | Circular motion, level road and banked road |
| 4.11 | A five step method for solving problems in mechanics |
The chapter closes with a Summary, eleven Points to Ponder and 23 exercises. Read the Points to Ponder twice. Most NEET traps in this unit are written there in plain words. If you are still building your Class 11 base, our NEET 2027 Class 11 study plan shows where this chapter fits in the year.
What is Newton's first law of motion?
Newton's first law of motion says every body stays at rest or in uniform motion in a straight line unless an external force compels it to change that state. NCERT also puts it in one line: if the net external force on a body is zero, its acceleration is zero. The property behind it is inertia, which NCERT defines as resistance to change.
Galileo got there first with inclined planes. A ball rolling down one smooth plane climbs the other to almost the same height, and as the second slope flattens it travels further. With a flat second plane and no friction, it would never stop. NASA's Glenn Research Center notes that Newton presented the three laws in 1686 in the Principia.
Here is the reasoning NCERT wants you to use, and it is the opposite of what most students write. For a book at rest on a table, do not say "weight equals normal force, so the book is at rest". Say "the book is at rest, so the net force is zero, so the normal force must equal the weight". The state of motion tells you about the forces, not the other way round. The same law explains why you lurch backward when a bus starts suddenly: friction moves your feet with the bus, while the rest of your body stays put for a moment.
What does the second law of motion really say?
The second law of motion says the rate of change of momentum of a body is proportional to the applied force and happens in the direction of that force. With the constant set to one, F = dp/dt, and for a fixed mass F = ma. One newton is the force that gives 1 kg an acceleration of 1 m s⁻², so 1 N = 1 kg m s⁻².
Four properties from NCERT carry most of the questions:
- It contains the first law. F = 0 gives a = 0.
- It is a vector law. It holds separately along x, y and z, which is why a projectile keeps its horizontal velocity while gravity changes only the vertical part.
- F is the net external force. For a system, internal forces drop out and a is the acceleration of the centre of mass.
- It is local in time. A stone dropped from an accelerating train has no horizontal acceleration the instant after release. It carries no memory of the train's push.
NCERT's Example 4.2 shows the size of forces involved. A 0.04 kg bullet at 90 m s⁻¹ stops in 60 cm of wood. The retardation is 6750 m s⁻², so the average resistive force is 0.04 × 6750 = 270 N.
What is impulse?
Impulse is force multiplied by the time it acts, and it equals the change in momentum. It is the right tool when a large force acts briefly and neither the force nor the time is easy to measure. In NCERT's Example 4.4, a 0.15 kg ball comes in at 12 m s⁻¹ and goes straight back at the same speed: the impulse is 0.15 × 12 minus (-0.15 × 12) = 3.6 N s, towards the bowler. A fielder who pulls the hands back while catching makes the stopping time longer, so the same impulse needs a smaller force.
Why don't action and reaction cancel?
Action and reaction never cancel because they act on different bodies. Newton's third law says that to every action there is an equal and opposite reaction; NCERT restates it as "force on A by B is equal and opposite to the force on B by A". The two forces are simultaneous, so neither is the cause of the other, and either one can be called the action.
NCERT's Example 4.12 is the cleanest test of this idea. A 2 kg block on a soft floor carries a 25 kg iron cylinder, and the floor gives way so both move down at 0.1 m s⁻². Taking g = 10 m s⁻², the weight of the system is 270 N, but the normal force from the floor is only 270 minus 27 × 0.1 = 267.3 N. Weight and normal force are on the same body and are not equal here, so they were never a third law pair. The real pairs are the earth on the system and the system on the earth, and the floor on the system and the system on the floor.
How does conservation of momentum follow?
Conservation of momentum follows from the second and third laws together: the total momentum of an isolated system stays constant. When a gun fires, the force on the bullet and the force on the gun are equal and opposite and act for the same short time, so their momentum changes are equal and opposite. NCERT stresses that this holds for elastic and inelastic collisions alike.
Try NCERT's Exercise 4.19 yourself. A 0.020 kg shell leaves a 100 kg gun at 80 m s⁻¹. Momentum before is zero, so 0.020 × 80 = 100 × v, which gives a recoil speed of 0.016 m s⁻¹ (our arithmetic).
Equilibrium of concurrent forces, in one rule
Equilibrium of concurrent forces means the vector sum of all the forces acting at one point is zero, so the particle is at rest or moving uniformly. For three forces, any two add up to a force equal and opposite to the third, and the three can be drawn as the sides of a closed triangle. Along each axis, the components add to zero.
In NCERT's Example 4.6, a 6 kg mass hangs from a rope and a 50 N horizontal force pulls the rope's midpoint sideways. The lower tension is 6 × 10 = 60 N. At the midpoint, T1 cos θ = 60 N and T1 sin θ = 50 N, so tan θ = 5/6 and θ is about 40 degrees from the vertical. The upper tension works out at about 78.1 N (our arithmetic). Notice that the rope's length never entered the answer.
How does friction work in NEET problems?
Friction is the component of the contact force parallel to the surfaces, and it opposes relative motion, impending or actual. Static friction is the one students get wrong. It is zero when nothing pushes, it grows to match the push, and it has a ceiling, (fs)max = µsN. Once sliding starts, kinetic friction fk = µkN takes over, and experiments show µk is less than µs. Both are independent of the area of contact.
| NCERT example | Data | Result |
|---|---|---|
| 4.7 Box on a train floor | µs = 0.15, g = 10 m s⁻² | Maximum train acceleration µsg = 1.5 m s⁻² |
| 4.8 Block on a tilting plane | Slips at θ = 15° | µs = tan 15° = 0.27, whatever the mass |
| 4.9 Hanging block and trolley | 3 kg block, 20 kg trolley, µk = 0.04 | a = 0.96 m s⁻², tension 27.1 N |
NCERT adds two facts that are easy to examine. Rolling friction is smaller than static or sliding friction, "even by 2 or 3 orders of magnitude" for the same weight, which is why the wheel and ball bearings matter. And friction is not always the enemy: you walk, and a car accelerates, only because static friction pushes you forward.
How do cars take turns on level and banked roads?
A car takes a turn because some real force supplies the centripetal force mv²/R. On a level road that force is static friction, so v² ≤ µsRg and the top speed is √(µsRg), independent of the car's mass. On a road banked at angle θ, the horizontal part of the normal force helps, so the safe speed rises. With no friction at all, the optimum speed is vo = √(Rg tan θ); with friction, v max = √(Rg (µs + tan θ)/(1 - µs tan θ)).
NCERT's Example 4.10 shows the level road case failing. A cyclist at 18 km/h (5 m s⁻¹) takes a 3 m turn with µs = 0.1. Here µsRg = 2.94 m² s⁻², but v² = 25 m² s⁻², so the cyclist slips. Example 4.11 then takes a 300 m race track banked at 15° with µs = 0.2 and gives the optimum speed as 28.1 m s⁻¹. The chart puts that next to the other two speeds for the same track.
One more line from the same section is a favourite for statement questions: below the optimum speed, friction on a banked road acts up the slope, and a car can be parked on the bank only if tan θ ≤ µs.
What does a weighing scale read in a lift?
A weighing scale reads the normal force, not your weight. That is the whole idea behind lift questions, and NCERT's Exercise 4.13 is the standard version: a 70 kg man stands on a scale in a lift, with g = 10 m s⁻². Apply the second law to the man alone and the scale reading follows.
Notice the first bar. A lift moving at a steady 10 m s⁻¹ reads the same as a lift standing still, because the acceleration is zero. Speed never enters the answer, only acceleration does. That single point catches many students who have learnt the laws of motion as words but not as a habit.
How do you solve any laws of motion problem?
You solve a laws of motion problem by choosing a system, drawing every external force on it, and applying F = ma along convenient axes. NCERT's Section 4.11 lays out the method, and it works on pulleys, blocks on trolleys and connected bodies alike:
- Draw a diagram of the whole assembly: bodies, strings, supports, pulleys.
- Choose one convenient part of the assembly as your system.
- Draw a free body diagram showing only the forces on that system from everything else, including gravity. Leave out the forces your system exerts on others.
- Mark the forces you know with magnitude and direction, such as a tension along its string, and treat the rest as unknowns.
- If you need more equations, choose another part as the system and use the third law: if A feels F from B, B feels -F from A.
Test it on NCERT's Exercise 4.16: 8 kg and 12 kg masses hang over a frictionless pulley. The free body diagrams give a = (12 - 8) × 10 / 20 = 2 m s⁻² and a tension of 96 N (our arithmetic). Then do Exercise 4.15 both ways round and watch the tension change from 400 N to 200 N when the 600 N pull moves from the 10 kg body to the 20 kg body.
Eight traps in laws of motion questions
Most of these come straight from NCERT's Points to Ponder, which is why they keep turning up.
- Force and velocity need not point the same way. Force is always parallel to acceleration, not to velocity.
- Momentarily at rest does not mean zero force. At the top of its flight a ball has v = 0, but the force is still mg and the acceleration is g.
- ma is not an extra force. It is the effect of the net force, so never add it to a free body diagram.
- Centripetal force is not a new kind of force. Find the real one: tension, gravity, friction or a normal force component.
- Do not write fs = µsN by reflex. Static friction equals µsN only at the point of slipping.
- mg = N is not the third law. It holds only in equilibrium, and in an accelerating lift it fails.
- Two equal and opposite forces on one body are never an action and reaction pair.
- Friction opposes relative motion, not motion. The box at rest on an accelerating train floor is pushed forward by friction.
A 7-day revision plan for laws of motion
- Day 1: Sections 4.1 to 4.4 and Example 4.1. Write the first law in NCERT's one line form and explain the book on the table both ways.
- Day 2: Second law and impulse. Examples 4.2 to 4.4, then Exercises 4.5 to 4.10.
- Day 3: Third law and momentum. Example 4.5 and Exercises 4.17 to 4.20, finishing with the gun recoil.
- Day 4: Equilibrium and free body diagrams. Examples 4.6 and 4.12, then Exercises 4.13 to 4.16 without looking at this guide.
- Day 5: Friction. Examples 4.7 to 4.9 and the table above from memory.
- Day 6: Circular motion. Examples 4.10 and 4.11, Exercises 4.21 and 4.22, and all three road formulas with their conditions.
- Day 7: A timed mixed set of 20 questions, then sort every mistake under one of the eight traps.
Keep our NEET Physics formula sheet beside you for the formulas of later units, and use the timed sets in NEET Physics questions for Day 7. The bigger picture, including where mechanics sits in your year, is in our NEET 2027 Physics preparation strategy, and our guide on how to study NCERT for NEET 2027 shows how to read a chapter like this one. To check which topics are out of the current syllabus, see the NEET 2027 deleted syllabus.
On futureMBBS, the NEET 2027 chapter-wise weightage guide helps you place this unit in a revision order, the NEET syllabus overview keeps all the Physics units on one page, and its piece on time management for the NEET exam helps once you start doing full papers.
Want to see whether the laws of motion are where your Physics marks leak? NEETsheet's chapter-wise Physics practice and timed mocks show which trap you keep falling into, so your next revision goes there first. Start at neetsheet.in.
Sources
All links were checked on 5 November 2026.
- National Medical Commission (UGMEB), Syllabus for NEET (UG) 2026, public notice of 22 December 2025 (Physics Units 2, 3 and 4): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/01/202601081066816297.pdf
- National Testing Agency, Information Bulletin NEET (UG) 2026 (pattern: Physics 45 questions and 180 of 720 marks; plus four and minus one marking; three hour duration; syllabus FAQ): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/02/202602231394640855.pdf
- National Testing Agency, NEET (UG) public notices, checked on 5 November 2026 for any NEET (UG) 2027 notice: https://neet.nta.nic.in/public-notices/
- National Testing Agency, Examination Calendar, December 2026 cycle onwards (no NEET (UG) 2027 date listed up to April 2027): https://nta.ac.in/Download/Notice/ExamcalendarRevised.pdf
- NCERT, Physics Class 11 Part I, Chapter 4 Laws of Motion, reprint 2026-27 (sections 4.1 to 4.11; Examples 4.1 to 4.12; Points to Ponder; Exercises 4.1 to 4.23): https://ncert.nic.in/textbook/pdf/keph104.pdf
- NASA Glenn Research Center, Newton's Laws of Motion (1686, Principia): https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion/
Frequently Asked Questions
Newton's first law of motion says every body stays at rest, or keeps moving in a straight line at constant speed, unless an external force makes it change. NCERT puts it in one line: if the net external force on a body is zero, its acceleration is zero. The property behind it is inertia, a body's resistance to change.
Yes, as far as anyone can say today. Laws of Motion is Physics Unit 3 in the NMC syllabus for NEET (UG) 2026, and the 2026 bulletin says the syllabus did not change from the previous year. No NEET (UG) 2027 syllabus or notice had been published by 5 November 2026.
Laws of motion is a Class 11 topic. It is Chapter 4 of NCERT Physics Class 11 Part I (reprint 2026-27), right after Motion in a Plane. Kinematics from Chapters 2 and 3 is the base, so revise vectors, relative velocity and uniform circular motion first if they still feel shaky.
Pseudo force is not named in the NMC syllabus for NEET (UG) 2026, and the 2026-27 NCERT chapter on laws of motion never uses the term. NCERT solves lift and train problems from the ground frame with real forces only. You can still use a pseudo force as a shortcut, but every such question can be solved without one.
The three laws of motion are Newton's laws. First, a body keeps its state of rest or uniform straight line motion unless a net external force acts. Second, the net force equals the rate of change of momentum, F = dp/dt, or F = ma for a fixed mass. Third, forces come in equal and opposite pairs acting on two different bodies.
