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Laws of Motion NCERT Solutions, Complete PDF Content and Important Questions
NCERT Solutions for Class 11 Physics Chapter 4 PDF Download (2026) + Important Questions Welcome to ExamSpark's ultimate guide on Class 11 Physics Chapter 4: Laws of Motion . If you are preparing for your 2026 Board Exams, CUET, JEE, or NEET, this chapter is the absolute backbone of Mechanics. In Chapter 3, we studied how things move, but here we dive into why they move. We will break down Newton's Laws, friction, and circular motion into simple, easy-to-understand Hinglish so that you don't just memorize, but actually understand the concepts. Toh chalo, physics ke in fundamental rules ko master karte hain! By the end of this post, you'll be able to solve the trickiest Free Body Diagrams (FBDs) with confidence.
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ChapterLaws of Motion
SubjectPhysics
Class11
BoardCBSE / State Boards (NCERT Pattern)
DifficultyModerate to High (Concept-Heavy)
Exam Weightage7-9 Marks (Highly important for competitive exams)
Learning Objectives
- Understand the concepts of Inertia and Force.
- Apply Newton's Three Laws of Motion to real-world physical systems. Draw accurate Free Body Diagrams (FBDs) to.
- Solve complex numericals.
- Apply the Law of Conservation of Linear Momentum.
- Differentiate between static, kinetic, and rolling Friction.
- Solve problems based on dynamics of Circular Motion (like banking of roads).
Key Concepts, Definitions and Formulas
- Newton's First Law: A body continues to be in its state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force
- Momentum (p): The quantity of motion contained in a body
- Newton's Second Law: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force
- Impulse: A large force acting for a very short time interval
- Change in momentum Newton's Third Law: To every action, there is always an equal and opposite reaction. (
- Action and reaction act on different bodies). Friction: An opposing force that comes into play when one body moves or tends to move over the surface of another
- Centripetal Force: The force required to keep a body moving in a circular path
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Full NCERT Solutions
NCERT Q1: Give the magnitude and direction of the net force acting on a drop of rain falling down with a constant speed.
Answer: Net Force is Zero. Step-by-step explanation: According to Newton's Second Law, Force = mass x acceleration (F = ma). Since the raindrop is falling with a constant speed , its acceleration is zero (a = 0). Therefore, the net force acting on it is 0. The downward force of gravity is perfectly balanced by the upward viscous drag (air resistance).
NCERT Q2: Give the magnitude and direction of the net force acting on a cork of mass 10 g floating on water.
Answer: Net Force is Zero. Step-by-step explanation: The cork is floating, meaning it is in a state of equilibrium. The downward force of gravity (weight of the cork) is exactly balanced by the upward buoyant force exerted by the water. Since there is no acceleration, the net force is zero.
NCERT Q3: Give the magnitude and direction of the net force acting on a kite skillfully held stationary in the sky.
Answer: Net Force is Zero. Step-by-step explanation: The keyword here is stationary . Because the kite is at rest, its velocity and acceleration are both zero. According to Newton's First Law, if a body is at rest, the net external force acting on it must be zero.
NCERT Q4: Give the magnitude and direction of the net force acting on a car moving with a constant velocity of 30 km/h on a rough road.
Answer: Net Force is Zero. Step-by-step explanation: The car is moving with a constant velocity , which means acceleration is zero. Even though the road is rough and friction is present, the forward force provided by the car's engine exactly cancels out the backward force of friction. Hence, net force = ma = 0.
NCERT Q5: Give the magnitude and direction of the net force acting on a high-speed electron in space far from all material objects, and free of electric and magnetic fields.
Answer: Net Force is Zero. Step-by-step explanation: Since the electron is far from all material objects, gravitational force is negligible. It is also free of electric and magnetic fields, meaning no electromagnetic forces are acting on it. With no external forces present, the net force is zero.
NCERT Q6: A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble (a) during its upward motion, (b) during its downward motion, (c) at the highest point.
Answer: Step-by-step explanation: In all three cases, the only force acting on the pebble (ignoring air resistance) is the force of gravity. Magnitude: F = mg = 0.05 kg x 9.8 m/s2 = 0.49 N Direction: Vertically downward. Whether the pebble is going up, coming down, or momentarily stopped at the highest point, gravity always acts downwards.
NCERT Q7: A constant force acting on a body of mass 3.0 kg changes its speed from 2.0 m/s to 3.5 m/s in 25 s. The direction of the motion of the body remains unchanged. What is the magnitude and direction of the force?
Answer: Step-by-step explanation: Given: Mass (m) = 3.0 kg Initial velocity (u) = 2.0 m/s Final velocity (v) = 3.5 m/s Time (t) = 25 s First, find acceleration (a) using the first equation of motion: v = u + at 3.5 = 2.0 + a(25) 1.5 = 25a a = 1.5 / 25 = 0.06 m/s2 Now, calculate Force (F): F = ma F = 3.0 x 0.06 = 0.18 N Direction: Since the speed is increasing, the force acts along the direction of motion. (Note: Download our full PDF guide at the end of the post for detailed mathematical derivations of Questions 8 to 23!)
Extra Important Questions for 2026 Exams
Important Q1: Physical independence of force is a consequence of: a) First law of motion b) Second law of motion c) Third law of motion d) All of these
Answer: (a) First law of motion (Law of Inertia).
Important Q2: A passenger in a moving bus is thrown forward when the bus suddenly stops. This is explained by: a) Inertia of rest b) Inertia of motion c) Inertia of direction d) Newton's third law
Answer: (b) Inertia of motion.
Important Q3: The coefficient of static friction depends on: a) Area of contact b) Nature of surfaces in contact c) Normal reaction d) Both b and c
Answer: (b) Nature of surfaces in contact. (2-3 Marks)
Important Q4: Why are shock absorbers used in cars and motorcycles?
Answer: Shock absorbers increase the time interval over which the force of impact acts during a bump. Since Impulse = F x Delta t, increasing the time (Delta t) reduces the magnitude of force (F) experienced by the passengers.
Important Q5: Is it easier to pull or push a lawnmower? Explain.
Answer: It is easier to pull. When pulling, the vertical component of the applied force acts upwards, reducing the effective weight and normal reaction, which decreases friction. When pushing, the vertical component adds to the weight, increasing friction.
Important Q6: State the Law of Conservation of Linear Momentum.
Answer: It states that if no external force acts on a system of particles, the total linear momentum of the system remains constant.
Important Q7: Why does a gun recoil when fired?
Answer: Due to the conservation of momentum. Before firing, the total momentum is zero. When the bullet moves forward with a certain momentum, the gun must move backward with an equal and opposite momentum to keep the total momentum zero.
Important Q8: Define angle of friction and angle of repose.
Answer: Angle of friction is the angle the resultant of normal reaction and limiting friction makes with the normal reaction. Angle of repose is the minimum angle of an inclined plane at which a body placed on it just begins to slide down. Both are numerically equal ($ \theta = \tan^{-1}\mu $). (5 Marks)
Important Q12: Assertion: Friction is a non-conservative force. Reason: Work done against friction in a closed path is zero.
Answer: Assertion is True, but Reason is False. Work done against friction in a closed path is NOT zero, which is exactly why it is non-conservative.
Important Q13: Assertion: An unbanked curve requires more reliance on friction to prevent skidding. Reason: Banking provides a horizontal component of normal force that acts as centripetal force.
Answer: Both A and R are true, and R is the correct explanation of A. 14 &
Important Q15: Read the passage and
Answer: A block of mass 10 kg is placed on a rough horizontal surface having coefficient of static friction 0.5. A horizontal force of 40 N is applied on the block. (Take g = 10 m/s2). Q14. What is the limiting friction? Answer: fs(max) = s x N = s x mg = 0.5 x 10 x 10 = 50 N. Q15. Will the block move? What is the friction force acting on it? Answer: The applied force (40 N) is less than the limiting friction (50 N). Therefore, the block will NOT move. The friction acting on it will self-adjust to equal the applied force, so friction = 40 N.
FAQ Section
Is Chapter 4 Laws of Motion important for Class 11 boards?
Yes, it is extremely important. It carries a high weightage and forms the basic foundation for upcoming chapters like Work, Energy & Power, and Rotational Motion.
Where can I download the Class 11 Physics Chapter 4 NCERT PDF?
You can download the official NCERT PDF directly from the NCERT website or through the direct download link provided at the top of this ExamSpark page.
Which topics are most frequently asked in the 2026 exams?
Banking of roads, conservation of linear momentum (gun recoil problems), and connected body problems (pulley and string) are board favorites.
How to solve friction numericals easily?
Always check if the applied force is greater than the maximum static friction ($\mu_s N$). If it is less, the object doesn't move, and friction equals the applied force. If it's greater, the object moves, and you use kinetic friction ($\mu_k N$).
Is the syllabus for Laws of Motion reduced for 2026?
CBSE rationalized the syllabus recently, but the core concepts of Newton's Laws, Momentum, and Friction remain untouched. Always stick to the latest rationalized NCERT book
More Class 11 Physics Chapters
Ch 1: Units and MeasurementsCh 2: Motion in a Straight LineCh 3: Motion in a PlaneCh 4: Laws of MotionCh 5: Work, Energy and PowerCh 6: System of Particles and Rotational MotionCh 7: GravitationCh 8: Mechanical Properties of SolidsCh 9: Mechanical Properties of FluidsCh 10: Thermal Properties of MatterCh 11: ThermodynamicsCh 12: Kinetic TheoryCh 13: OscillationsCh 14: Waves