Schooling · Class 11 · Physics · Laws of Motion · Chapter on a page
CBSE Class 11 · Physics · NCERT Physics Part-I
Laws of Motion: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Newton's First Law (Law of Inertia)
- Newton's Second Law
- Newton's Third Law
- Momentum
- Friction
- Free-body diagram
- Equilibrium
- Formulas and facts
- F_net = ma
- p = mv
- W = mg
- f_s ≤ μ_s N
- f_k = μ_k N
- Centripetal acceleration
- For a body on an incline at angle θ
- Worked examples
- Applying Newton's second law
- Friction on a horizontal surface
- Two blocks connected by a string
- Watch out for
- Confusing mass and weight
- Applying Newton's third law forces…
- Forgetting that static friction…
- Using weight instead of normal force…
- Ignoring direction when using F = ma
What this chapter is about
This chapter introduces the fundamental principles that govern why objects move the way they do. While earlier chapters described motion using kinematics (how things move), this chapter answers the deeper question: what causes motion to change? The answer lies in forces and the three laws formulated by Isaac Newton in the 17th century.
Key ideas
- Newton's First Law (Law of Inertia): An object remains at rest or moves with constant velocity unless acted upon by a net external force. This defines inertia as the natural tendency to resist changes in motion.
- Newton's Second Law: The net force on an object equals the rate of change of its momentum. For constant mass, F = ma, where F is in newtons, m in kilograms, and a in metres per second squared.
- Newton's Third Law: When object A exerts a force on object B, object B exerts an equal and opposite force on object A. These forces act on different objects and never cancel each other.
- Momentum: Defined as p = mv, momentum is a vector quantity measured in kg m/s. The law of conservation of momentum states that total momentum of an isolated system remains constant.
- Friction: A contact force opposing relative motion between surfaces. Static friction (f_s ≤ μ_s N) prevents motion; kinetic friction (f_k = μ_k N) acts during sliding. Here μ is the coefficient of friction and N is the normal force.
- Free-body diagram: A sketch showing all forces acting on a single object, essential for correctly applying Newton's second law.
- Equilibrium: An object is in equilibrium when the net force on it is zero, meaning it either remains at rest or moves with constant velocity.
- Circular motion and centripetal force: An object moving in a circle requires a net inward force of magnitude mv²/r to maintain its curved path.
Formulas and facts to remember
- Newton's second law: F_net = ma (force in N, mass in kg, acceleration in m/s²)
- Momentum: p = mv; impulse = change in momentum = F × Δt
- Weight: W = mg, where g ≈ 9.8 m/s² near Earth's surface
- Static friction: f_s ≤ μ_s N (maximum value is μ_s N just before sliding begins)
- Kinetic friction: f_k = μ_k N (constant during sliding, typically μ_k < μ_s)
- Centripetal acceleration: a_c = v²/r directed toward the centre
- For a body on an incline at angle θ: component of weight along incline = mg sin θ; normal force N = mg cos θ
- Conservation of momentum: In the absence of external forces, m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Common mistakes
- Confusing mass and weight → Mass is the quantity of matter (kg); weight is the gravitational force (N) given by mg.
- Applying Newton's third law forces to the same object → Action and reaction act on different objects; they do not cancel when analysing one body.
- Forgetting that static friction adjusts itself → It only equals μ_s N at the point of impending motion; before that, it matches the applied force.
- Using weight instead of normal force to calculate friction on an incline → On an incline, N = mg cos θ, not mg.
- Ignoring direction when using F = ma → Force and acceleration are vectors; signs must reflect direction consistently.
Quick revision
- First law: no net force means no change in velocity.
- Second law: F_net = ma; use a free-body diagram first.
- Third law: equal and opposite forces on two different bodies.
- Friction opposes relative motion; f ≤ μN for static, f = μN for kinetic.
- Momentum is conserved when no external force acts.
- For circular motion, a centripetal force mv²/r must be provided by real forces like tension, gravity, or friction.