Newton's Laws — Set 3

Correct Answer: C. Centripetal Force

• **Centripetal Force** = Centripetal force is the net force directed toward the center of a circular path, required to continuously change the direction of velocity for circular motion. • **F_c = mv²/r** — without centripetal force, the object would move in a straight line (Newton's First Law). • 💡 Wrong-option analysis: Frictional Force: friction can provide centripetal force for a car turning, but it is not the general answer — centripetal force is the category; Gravitational Force: gravity can act as centripetal force for orbital motion but not generally; Centrifugal Force: this is a pseudo-force in the rotating frame, not a real force providing circular motion.

Correct Answer: C. mg

• **mg** = Weight is the gravitational force on an object: W = mg, where m is mass and g is acceleration due to gravity. • **W = mg** — weight is measured in newtons (N); mass is in kg and g ≈ 9.8 m/s². • 💡 Wrong-option analysis: m/g: this has units kg/(m/s²) = kg²s²/kg·m — not a force; m−g: dimensionally inconsistent — cannot subtract m/s² from kg; m+g: dimensionally inconsistent for the same reason.

Correct Answer: A. First Law

• **First Law** = Newton's First Law qualitatively defines force as any external agent that can change the state of rest or uniform motion of a body. • **qualitative definition** — the Second Law gives the quantitative relationship (F = ma); the First is the conceptual foundation. • 💡 Wrong-option analysis: Second Law: the second law gives the quantitative formula F = ma; Third Law: third law describes action-reaction pairs; Law of Universal Gravitation: this is a separate law about gravitational attraction.

Correct Answer: B. Both Momentum and Kinetic Energy

• **Both Momentum and Kinetic Energy** = In a perfectly elastic collision, both the total linear momentum and the total kinetic energy of the system are conserved. • **elastic: ΔKE = 0** — in inelastic collisions momentum is conserved but kinetic energy is not (some converts to heat/sound). • 💡 Wrong-option analysis: Neither Momentum nor Kinetic Energy: momentum is always conserved in isolated collisions (elastic or inelastic); Momentum only: this describes an inelastic collision where KE is lost; Kinetic Energy only: kinetic energy cannot be conserved without momentum also being conserved in an isolated system.

Correct Answer: D. They are equal

• **They are equal** = Newton's Third Law: action and reaction forces are always equal in magnitude and opposite in direction. • **|F_action| = |F_reaction|** — they act on different bodies simultaneously; neither force is 'more powerful' than the other. • 💡 Wrong-option analysis: Reaction is greater: wrong — Newton's Third Law says they are exactly equal; They depend on mass: the magnitudes of action and reaction are always equal regardless of the masses involved; Action is greater: wrong — there is no preferred 'action' or 'reaction' in physics.

Correct Answer: C. Inertia

• **Inertia** = Inertia is the inherent property of a body that causes it to resist any change in its current state of motion or rest. • **inertia ∝ mass** — the First Law is also called the 'Law of Inertia'; more massive objects have greater inertia. • 💡 Wrong-option analysis: Acceleration: acceleration is the result of a net force acting on a body — it is a response to change, not a resistance to it; Force: force is the agent that causes change in motion; Velocity: velocity is a kinematic quantity describing the state of motion, not the property resisting its change.

Correct Answer: A. 1 m/s²

• **1 m/s²** = F = ma → a = F/m = 1 N / 1 kg = 1 m/s². • **1 N = 1 kg·m/s²** — by definition, one Newton accelerates one kilogram at exactly 1 m/s². • 💡 Wrong-option analysis: 0.1 m/s²: would require m = 10 kg for a 1 N force; 10 m/s²: would require m = 0.1 kg; 9.8 m/s²: this is g, relevant only when the 1 N force is gravity on ~0.1 kg, not the general case here.

Correct Answer: C. Third Law

• **Third Law** = Walking involves pushing backward on the ground (action); the ground pushes the person forward with an equal and opposite force (reaction) — Newton's Third Law. • **push backward → ground pushes forward** — on a frictionless surface this reaction cannot be generated and walking fails. • 💡 Wrong-option analysis: First Law: first law explains why a person continues moving at constant speed, not how propulsion begins; Second Law: second law gives force = ma but doesn't explain why the ground pushes back; None of these: Newton's Third Law clearly explains walking.

Correct Answer: D. Force

• **Force** = Force is a vector quantity because it has both magnitude (measured in Newtons) and a specific direction of action. • **F = ma (vector equation)** — both mass (scalar) and acceleration (vector) combine to give force as a vector. • 💡 Wrong-option analysis: Speed: speed is the magnitude of velocity — a scalar with no direction; Distance: distance is total path length — a scalar; Mass: mass has only magnitude (kg) and no direction.

Correct Answer: A. No reaction force can be generated

• **No reaction force can be generated** = Walking requires pushing backward on the ground; on a perfectly frictionless surface there is no grip, so no backward push is possible, and hence no forward reaction force. • **Newton's Third Law requires a reaction** — friction is the agent that allows the reaction force; without it, propulsion is impossible. • 💡 Wrong-option analysis: Gravity is absent: gravity still acts on a frictionless floor — the problem is the lack of friction, not gravity; Mass increases: mass does not change on a frictionless surface; Inertia is zero: inertia depends on mass, which does not change.