Laws of Motion Class 11 Physics

In this chapter, you will learn about Laws of Motion in Class 11 Physics, an important topic for your board exams, JEE, and NEET preparation. Master the concepts with our free resources below.

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Key Concepts

Newton's First Law (Law of Inertia): A body remains at rest or in uniform motion in a straight line unless acted upon by an external net force. Inertia is the property of a body to resist change in its state of motion.
Newton's Second Law: The rate of change of momentum of a body is directly proportional to the applied force and acts in the direction of the force: F = ma (or F = dp/dt).
Newton's Third Law: For every action there is an equal and opposite reaction. Action and reaction always act on different bodies.
Linear Momentum and its Conservation: Momentum p = mv. In the absence of external forces, the total momentum of a system is conserved. This principle explains recoil of guns and rocket propulsion.
Friction: Static friction (f_s ≤ μ_s N) opposes impending motion; kinetic friction (f_k = μ_k N) opposes sliding motion. Limiting friction = μ_s N; kinetic friction is always less than limiting friction.

Important Formulas

Newton's Second Law: F = ma = dp/dt

Impulse: J = F × Δt = Δp = m(v − u)

Law of Conservation of Momentum: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ (no external force)

Static Friction (max): f_s(max) = μ_s × N

Kinetic Friction: f_k = μ_k × N

Apparent Weight in lift (accelerating up): W' = m(g + a)

Apparent Weight in lift (accelerating down): W' = m(g − a)

Solved Numerical Examples

Example 1 — Impulse and Momentum

Problem: A cricket ball of mass 0.15 kg moving at 30 m/s is brought to rest by a fielder in 0.05 s. Find the average force exerted by the fielder on the ball.

Solution: Change in momentum = m(v − u) = 0.15 × (0 − 30) = −4.5 kg m/s.
Average force F = Δp/Δt = −4.5 / 0.05 = −90 N.
The magnitude of force exerted is 90 N (opposing the motion).

Example 2 — Friction on an Inclined Plane

Problem: A block of mass 5 kg rests on a rough inclined plane of angle 30°. The coefficient of static friction is 0.4. Find whether the block remains stationary. (g = 10 m/s²)

Solution: Component of gravity along the plane = mg sin30° = 5 × 10 × 0.5 = 25 N.
Normal force N = mg cos30° = 5 × 10 × 0.866 = 43.3 N.
Maximum static friction = μ_s × N = 0.4 × 43.3 = 17.3 N.
Since 25 N > 17.3 N, the block slides down the incline.

Daily Practice Problems (DPP)

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Laws of Motion DPP Quiz

[ { "question": "A bullet of mass 0.04 kg moving with a speed of 90 m/s enters a heavy wooden block and is stopped after a distance of 60 cm. What is the average resistive force exerted by the block on the bullet?", "options": [ "270 N", "27 N", "2.7 N", "2700 N" ], "correct": 0 }, { "question": "A force of 5 N gives a mass m1 an acceleration of 8 m/s^2 and a mass m2 an acceleration of 24 m/s^2. What acceleration would it give if both the masses are tied together?", "options": [ "2 m/s^2", "4 m/s^2", "6 m/s^2", "8 m/s^2" ], "correct": 2 }, { "question": "According to Newton's third law of motion, action and reaction", "options": [ "always act on the same body", "always act on different bodies in opposite directions", "have same magnitude and direction", "act on either body at normal to each other" ], "correct": 1 }, { "question": "A lift of mass 1000 kg is moving upwards with an acceleration of 1 m/s^2. The tension developed in the string, which is connected to the lift, is (g = 9.8 m/s^2)", "options": [ "9800 N", "10000 N", "10800 N", "11000 N" ], "correct": 2 }, { "question": "A body of mass 2 kg is hung on a spring balance mounted vertically in a lift. If the lift descends with an acceleration equal to the acceleration due to gravity g, the reading on the spring balance will be", "options": [ "2 kg", "4 kg", "zero", "1 kg" ], "correct": 2 }, { "question": "Which of the following is not a conservative force?", "options": [ "Gravitational force", "Electrostatic force", "Magnetic force", "Frictional force" ], "correct": 3 }, { "question": "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. This is a statement of", "options": [ "Newton's first law", "Newton's second law", "Newton's third law", "Law of conservation of momentum" ], "correct": 1 }, { "question": "The apparent weight of a person in a lift is less than their true weight when the lift is", "options": [ "Moving up with uniform speed", "Moving down with uniform speed", "Moving up with uniform acceleration", "Moving down with uniform acceleration" ], "correct": 3 }, { "question": "A rocket works on the principle of conservation of", "options": [ "Mass", "Energy", "Linear momentum", "Angular momentum" ], "correct": 2 }, { "question": "Maximum value of static friction is called", "options": [ "Limiting friction", "Rolling friction", "Kinetic friction", "Normal friction" ], "correct": 0 } ]

Multiple Choice Questions (MCQs)

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Previous Year Questions (PYQs)

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Common Mistakes

Forgetting to convert units appropriately.
Misapplying the sign convention.
Ignoring vector components in calculations.