Laws of Motion — Class 11 MCQs with Answers

Laws of Motion — important questions & MCQs with answers (Class 11 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Newton's first law is also called law of:

   AAction-reactionBForceCInertiaDGravitation
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   Answer: Inertia

   Explanation: Newton's first law is the law of inertia: with zero net force a body keeps its state of motion, at rest or moving. Action–reaction pairing is the third law, and force/gravitation are separate laws — the usual mix-up here.

2. Q2 of 6Easy

   Newton's First Law states a body remains at rest or in uniform motion unless:

   AIt is aliveBAn external force acts on itCTime passesDGravity acts
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   Answer: An external force acts on it

   Explanation: Newton's first law (inertia) says a body stays at rest or keeps moving at constant velocity unless an external net force acts on it. It doesn't depend on being 'alive' or on time passing alone — only an unbalanced force changes its state.

3. Q3 of 6Medium

   Two carts (3 kg at 4 m/s and 2 kg at rest) on a smooth track. After collision (perfectly inelastic), velocity:

   

   A2.4 m/sB4 m/sC2 m/sD0 m/s
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   Answer: 2.4 m/s

   Explanation: Momentum conservation for a perfectly inelastic collision: m₁v₁ = (m₁+m₂)v_f, so 3 kg × 4 m/s = 5 kg × v_f gives v_f = 12/5 = 2.4 m/s. Dividing by only one cart's mass instead of the combined 5 kg is what produces the wrong 4 m/s distractor.

4. Q4 of 6Medium

   Ball of 200 g hits wall at 10 m/s and rebounds at 10 m/s. Impulse:

   

   A20 N·sB2 N·sC0D4 N·s
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   Answer: 4 N·s

   Explanation: Impulse equals the change in momentum: Δp = m(v_f − v_i) = 0.2 kg × (10 − (−10)) m/s = 4 N·s, taking the rebound velocity as negative since it reverses direction. Forgetting the sign flip on rebound, giving 10 − 10 = 0, is the common error.

5. Q5 of 6Hard

   Maximum static friction is:

   AmgBμ_k NCμ_s ND0
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   Answer: μ_s N

   Explanation: The maximum static friction a surface can provide is f_s,max = μ_s N, using the coefficient of static friction with the normal reaction N. Using μ_k (kinetic) instead gives the sliding friction value, which is smaller, not the maximum static limit.

6. Q6 of 6Hard

   Vehicle moving at 36 km/h on curve of radius 30 m. Coefficient of friction needed:

   A0.1B0.5C1/3D1
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   Answer: 1/3

   Explanation: The minimum friction coefficient needed to turn is μ ≥ v²/(rg); converting 36 km/h to 10 m/s first, μ ≥ (10)²/(30 × 10) = 100/300 = 1/3 ≈ 0.33. Forgetting to convert km/h to m/s before squaring is the single most common trap here.

Laws of Motion notes: key concepts (Class 11 Physics)

Newton's three laws and the dynamics that follow from them: inertia, force as the rate of change of momentum, action-reaction pairs, conservation of linear momentum, impulse, friction in its static and kinetic forms, motion on inclines, and the circular-motion applications of banking and the level curve.

Inertia

A body's resistance to any change in its state of rest or uniform motion. Its measure is mass alone — inertia does not depend on speed, shape or location.

Newton's first law

A body stays at rest or in uniform straight-line motion unless a net external force acts. It defines both force and the inertial frames in which the laws hold.

Newton's second law

The net force equals the rate of change of momentum, reducing to F = ma when mass is constant. This is the equation that actually solves dynamics problems.

Newton's third law

Every force is one half of a pair, equal in magnitude and opposite in direction. The two forces act on DIFFERENT bodies, which is why they never cancel each other.

Linear momentum

The product of mass and velocity, p = mv. It is a vector, and Newton's second law is really a statement about how it changes.

Conservation of momentum

For a system with no net external force, total momentum stays constant. This survives collisions and explosions where kinetic energy does not.

Impulse

The product of force and the time for which it acts, equal to the change in momentum. It explains why airbags and follow-through in sport reduce the peak force.

Free-body diagram

A sketch of one chosen body with every external force on it drawn and nothing else. Almost every mechanics error traces back to a missing or invented force here.

Normal force

The contact force perpendicular to a surface. It equals mg only in the simplest case — on an incline it is mg cos θ, and in a lift it changes with the acceleration.

Tension

The pull transmitted along a string. In an ideal massless, inextensible string over a frictionless pulley the tension is the same throughout.

Static friction

A self-adjusting force that exactly balances the applied force up to a maximum of μ_s N. Below that limit it is NOT equal to μ_s N.

Kinetic friction

The roughly constant force opposing relative sliding, equal to μ_k N. It is generally slightly smaller than the limiting static friction.

Angle of repose

The incline angle at which a body just begins to slide, where tan θ equals the coefficient of static friction. It gives a neat way to measure μ_s.

Motion on an inclined plane

The weight resolves into mg sin θ down the slope and mg cos θ into it, so a smooth incline gives acceleration g sin θ regardless of mass.

Centripetal force

The net inward force needed to keep a body on a circular path. It is never a new force — it is supplied by tension, friction, gravity or a normal force.

Banking of roads

Tilting a curve so the normal force supplies part of the centripetal force, letting vehicles turn safely without relying entirely on friction.