Newton's Laws — Set 6

Correct Answer: B. Second Law

• **Second Law** = F = ma is the mathematical statement of Newton's Second Law, relating net force, mass, and acceleration. • **F = ma** — this is the quantitative foundation of classical mechanics, from which many other equations are derived. • 💡 Wrong-option analysis: Fourth Law: there is no Newton's Fourth Law of Motion; First Law: the first law is F = 0 → constant velocity (a qualitative principle); Third Law: third law states action = −reaction, not F = ma.

Correct Answer: A. Newton's Third Law

• **Newton's Third Law** = 'For every action, there is an equal and opposite reaction' is the standard statement of Newton's Third Law. • **Third Law** — the forces act on different bodies simultaneously; they never cancel each other. • 💡 Wrong-option analysis: Newton's Second Law: second law is F = ma — about force, mass, and acceleration; Newton's First Law: first law is the law of inertia — about constant velocity without force; Law of Inertia: this is another name for Newton's First Law, not the Third.

Correct Answer: D. 98 N

• **98 N** = Weight = mg = 10 kg × 9.8 m/s² = 98 N. • **W = mg = 10×9.8 = 98 N** — weight is measured in newtons, not kilograms; 100 N is the approximate value using g ≈ 10. • 💡 Wrong-option analysis: 100 kg: kilograms are unit of mass, not weight (force); 1 N: would require m ≈ 0.1 kg for g = 9.8 m/s²; 10 N: would require m = 1 kg, not 10 kg.

Correct Answer: A. First Law

• **First Law** = Newton's First Law is known as the 'Law of Inertia' because it describes the tendency of objects to resist any change in their state of motion. • **First Law = Law of Inertia** — inertia is the central concept: bodies at rest stay at rest; moving bodies continue at constant velocity. • 💡 Wrong-option analysis: Third Law: the third law is about action-reaction pairs; Law of Momentum: this is not an official name for any Newton's law; Second Law: the second law is sometimes called the 'Real Law of Motion' due to its quantitative nature.

Correct Answer: B. Momentum

• **Momentum** = Newton's Second Law in general form: F = dp/dt; force is the rate of change of momentum. • **F = dp/dt** — for constant mass this reduces to F = ma; the SI unit of momentum is kg·m/s. • 💡 Wrong-option analysis: Mass: mass does not change due to a force in classical mechanics; Velocity: rate of change of velocity is acceleration (dv/dt = a), not force directly; Acceleration: rate of change of acceleration is 'jerk', not force.

Correct Answer: D. Third Law

• **Third Law** = When a bullet is fired forward (action), the gun experiences an equal and opposite backward force (reaction) — this is Newton's Third Law, causing recoil. • **conservation of momentum** — bullet gains forward momentum; gun gains equal backward momentum; total remains zero. • 💡 Wrong-option analysis: Law of Gravitation: gravitation is the attractive force between masses — unrelated to gun recoil; First Law: first law is about inertia — while inertia is involved, the recoil mechanism is action-reaction (Third Law); Second Law: second law gives the magnitude of the recoil acceleration but doesn't explain why the gun moves backward.

Correct Answer: A. Mass

• **Mass** = The inertia of an object is directly proportional to its mass; a more massive object is harder to accelerate or decelerate. • **inertia ∝ mass** — a truck (large mass) has much more inertia than a bicycle (small mass). • 💡 Wrong-option analysis: Speed: speed is a kinematic state, not a determinant of inertia; Shape: shape affects aerodynamics and surface friction but not inertia; Volume: two objects with the same volume can have different masses (and hence different inertia) if their densities differ.

Correct Answer: D. Reduce force

• **Reduce force** = By pulling hands back while catching, the goalkeeper increases Δt — the time for the ball's momentum to reach zero; by F = Δp/Δt, greater Δt gives smaller average force, protecting the hands. • **F = Δp/Δt** — same principle as a cricketer or a car crumple zone: extend time to reduce force. • 💡 Wrong-option analysis: Decrease time: decreasing time would increase the impact force — the opposite of what is desired; Increase momentum: the ball's momentum is determined by its mass and speed at the time of catching — the goalkeeper cannot increase it; Increase force: increasing force would cause more pain and risk of injury.

Correct Answer: C. Low Friction

• **Low Friction** = On a slippery road, friction is reduced, so it is difficult to push backward on the ground with enough force to generate a forward reaction force for walking. • **Newton's Third Law requires friction** — low friction means the ground cannot provide adequate reaction to propel a person forward. • 💡 Wrong-option analysis: Low Inertia: inertia depends on mass, which does not change on a slippery road; High Friction: high friction makes walking easier, not harder; High Gravity: gravity does not change on a slippery road.

Correct Answer: A. Every action has an equal and opposite reaction

• **Every action has an equal and opposite reaction** = This is the precise statement of Newton's Third Law: for every force exerted by A on B, B exerts an equal and opposite force on A. • **Third Law** — forces are equal in magnitude, opposite in direction, and act on different bodies simultaneously. • 💡 Wrong-option analysis: A body at rest stays at rest unless acted upon: this is Newton's First Law (Law of Inertia); Gravitational force is proportional to mass: this is part of Newton's Law of Universal Gravitation; Force equals mass times acceleration: this is Newton's Second Law (F = ma).