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Laws of Motion – Class 11 Physics notes, important questions and MCQs

NCERT Class 11 Physics, chapter 4. Notes, key terms, important questions with answers and MCQs.

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Chapter overview

Newton's three laws explain how forces change motion. Momentum, impulse, friction and circular motion follow from them.

Newton's laws

First law (inertia): a body stays at rest or in uniform motion unless an external force acts.
Second law: F = dp/dt = ma; impulse = FΔt = Δp.
Third law: every action has an equal and opposite reaction acting on different bodies; momentum is conserved when no external force acts.

Friction and circular motion

Static friction fₛ ≤ µₛN; kinetic friction fₖ = µₖN; µₖ < µₛ usually.
Centripetal force = mv²/r; on a level road it is provided by friction: v_max = √(µrg).
Banked road: v_max = √[rg(µ + tan θ)/(1 − µ tan θ)]; without friction v = √(rg tan θ).

Key terms

Inertia: tendency of a body to resist change in its state of motion
Impulse: product of force and time, equal to change in momentum
Free-body diagram: a sketch showing all external forces acting on one chosen body

Formulas

F = ma
Impulse = FΔt = Δp
v_max = √(µrg)

Important questions with answers

Why does a cricketer lower his hands while catching a ball?2 marks

To increase the time of impact. Since F = Δp/Δt, a longer time reduces the force on his hands.

State the law of conservation of linear momentum and derive it from Newton's second and third laws.3 marks

When no external force acts, the total momentum of a system remains constant. For two colliding bodies, F₁₂ = −F₂₁ (third law). Using F = Δp/Δt over the same time: Δp₁ = −Δp₂, so Δp₁ + Δp₂ = 0 and p₁ + p₂ stays constant.

Two masses 4 kg and 6 kg are connected by a light string over a frictionless pulley (Atwood machine). Find the acceleration and tension (g = 10 m/s²). Also find the maximum safe speed on a level curve of radius 100 m if µ = 0.4.5 marks

For the heavier mass: 6g − T = 6a; for the lighter: T − 4g = 4a. Adding: 2g = 10a, a = 2 m/s². T = 4(10 + 2) = 48 N. On a level curve friction gives the centripetal force: µmg = mv²/r, v = √(µrg) = √(0.4 × 100 × 10) = 20 m/s (72 km/h).

Quick revision

Action and reaction act on different bodies.
Momentum conserved without external force.
Friction opposes relative motion.

Exam tips

In a lift accelerating up, apparent weight = m(g + a); down, m(g − a).
Always draw a free-body diagram for each body separately before applying F = ma.

Practice MCQs

Newton's first law is also called the law of
1. momentum
2. inertia
3. action
4. gravitation
Show answer
Answer: (b) inertia. Inertia.
A force of 10 N on a 2 kg mass gives acceleration
1. 20 m/s²
2. 5 m/s²
3. 12 m/s²
4. 0.2 m/s²
Show answer
Answer: (b) 5 m/s². F/m.
A gun recoils on firing because of
1. first law
2. conservation of momentum
3. friction
4. gravity
Show answer
Answer: (b) conservation of momentum. Momentum conserved.
Apparent weight of a 50 kg person in a lift moving up with a = 2 m/s² (g = 10) is
1. 400 N
2. 600 N
3. 500 N
4. 100 N
Show answer
Answer: (b) 600 N. 50 × 12.
Two blocks of 2 kg and 3 kg on a smooth surface are pushed by 10 N. Their acceleration is
1. 5 m/s²
2. 2 m/s²
3. 3.3 m/s²
4. 1 m/s²
Show answer
Answer: (b) 2 m/s². 10/5.
Max speed on a level curve of radius 50 m with µ = 0.2 (g = 10) is
1. 5 m/s
2. 20 m/s
3. 100 m/s
4. 10 m/s
Show answer
Answer: (d) 10 m/s. √(0.2 × 50 × 10).
A 0.1 kg ball hits a wall at 10 m/s and rebounds at 10 m/s. Change in momentum is
1. 2 kg m/s
2. 1 kg m/s
3. 0
4. 0.5 kg m/s
Show answer
Answer: (a) 2 kg m/s. 0.1 × 20.
A 2 kg block rests on a rough surface (µₛ = 0.5, g = 10). The minimum horizontal force to move it is
1. 2 N
2. 5 N
3. 10 N
4. 20 N
Show answer
Answer: (c) 10 N. µₛmg.

All chapters of Class 11 Physics

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.