Newton's Laws — Set 1

Correct Answer: A. Mass

• **Mass** = Mass is the quantitative measure of inertia; the greater the mass, the greater the resistance to changes in motion. • **inertia ∝ mass** — inertia is an intrinsic property of matter, independent of gravity and location. • 💡 Wrong-option analysis: Volume: two objects can have the same volume but different masses and hence different inertia; Density: density is mass per volume — inertia depends on total mass, not density alone; Weight: weight = mg and varies with g (e.g. on the Moon), but inertia (mass) remains the same.

Correct Answer: B. Newton's First Law

• **Newton's First Law** = Newton's First Law states that an object at rest remains at rest and a moving object continues in uniform straight-line motion unless acted upon by an external net force. • **Law of Inertia** — this is also called the 'Law of Inertia'; it defines the concept of force qualitatively. • 💡 Wrong-option analysis: Newton's Second Law: this law gives the quantitative relationship F = ma; Law of Gravitation: this describes gravitational attraction between masses; Newton's Third Law: this deals with action-reaction pairs.

Correct Answer: C. Acceleration

• **Acceleration** = In F = ma, 'a' represents the acceleration produced by the net force acting on the mass m. • **a = F/m** — acceleration is inversely proportional to mass for a given force; doubling mass halves acceleration. • 💡 Wrong-option analysis: Amplitude: amplitude relates to oscillatory motion, not Newton's second law; Area: area is a geometric quantity, unrelated to the force equation; Altitude: altitude is a height measurement, not a dynamical variable in F = ma.

Correct Answer: D. Inertia of Rest

• **Inertia of Rest** = When a bus starts suddenly, the lower body moves with the bus while the upper body tends to remain at rest due to inertia of rest, causing passengers to fall backward. • **Newton's First Law** — the body resists the sudden change from rest; this is a classic daily-life example of inertia. • 💡 Wrong-option analysis: Centripetal Force: centripetal force is required for circular motion, not relevant here; Momentum: momentum is the product mv — while related to inertia, it does not explain the backward falling; Inertia of Motion: inertia of motion applies when a moving body tends to continue moving, e.g. when the bus suddenly stops.

Correct Answer: A. Two

• **Two** = Newton's Third Law always involves an interaction between exactly two distinct bodies — body A exerts force on B, and B exerts equal and opposite force on A. • **action–reaction pair** — these two forces act on different bodies and therefore never cancel each other. • 💡 Wrong-option analysis: Three: Newton's Third Law involves exactly two bodies, not three; One: a single body cannot exert a Newton's Third Law pair on itself; Four: the law describes a pairwise interaction between exactly two bodies.

Correct Answer: A. Momentum

• **Momentum** = Momentum p = mv is a vector quantity found by multiplying an object's mass (m) by its velocity (v). • **p = mv** — momentum is conserved in isolated systems; the SI unit is kg·m/s. • 💡 Wrong-option analysis: Work: work = force × displacement, not mass × velocity; Force: force = rate of change of momentum (F = dp/dt), not momentum itself; Power: power = work/time, unrelated to mass × velocity.

Correct Answer: A. Gravity

• **Gravity** = Gravity is a fundamental non-contact force that acts over a distance without physical contact and fully obeys Newton's laws. • **F = Gm₁m₂/r²** — according to Newton's second law, gravity produces the familiar acceleration g ≈ 9.8 m/s² near Earth. • 💡 Wrong-option analysis: Air Resistance: this is a contact force (fluid friction) requiring physical contact with air molecules; Tension: tension acts through a string or rope — a contact force; Friction: friction requires surfaces to be in direct contact — it is a contact force.

Correct Answer: B. It moves with constant velocity

• **It moves with constant velocity** = Newton's First Law: if the net force is zero, there is no acceleration, so the object's velocity remains unchanged in magnitude and direction. • **F_net = 0 → a = 0 → v = constant** — this is the condition for equilibrium of a moving body. • 💡 Wrong-option analysis: It accelerates: acceleration requires nonzero net force; It stops immediately: stopping requires a net braking force; It slows down gradually: slowing down also requires a net retarding force.

Correct Answer: B. Third Law

• **Third Law** = A swimmer pushes water backward (action); the water pushes the swimmer forward with an equal and opposite force (reaction) — Newton's Third Law. • **action = reaction** — the reaction force from the water propels the swimmer; without water to push against, swimming is impossible. • 💡 Wrong-option analysis: First Law: first law relates to inertia and constant velocity — not to the propulsive mechanism; Law of Conservation of Mass: this is a chemistry principle; Second Law: second law gives force = ma, but it doesn't explain why the water pushes back.

Correct Answer: C. It is halved

• **It is halved** = From F = ma: a = F/m; if mass doubles and force stays constant, a = F/(2m) = half the original acceleration. • **a ∝ 1/m** — Newton's Second Law: for constant force, acceleration is inversely proportional to mass. • 💡 Wrong-option analysis: It doubles: doubling applies when force doubles, not when mass doubles; It quadruples: quadrupling would occur if mass were reduced to one quarter; It remains the same: if mass changes, acceleration changes unless force also changes proportionally.