Newton's Laws — Set 2

Correct Answer: B. Friction

• **Friction** = Friction is the contact force that acts between two surfaces in contact, always opposing their relative motion or tendency to move. • **opposes relative motion** — kinetic friction acts during sliding; static friction acts to prevent sliding from starting. • 💡 Wrong-option analysis: Electrostatic Force: this acts between charged objects at a distance, not specifically between surfaces in contact; Normal Force: the normal force acts perpendicular to the surface, not along it opposing motion; Magnetic Force: this acts between magnetic poles or current-carrying conductors.

Correct Answer: C. Normal Force

• **Normal Force** = The normal force is the perpendicular contact force exerted by a surface on an object resting on it, preventing the object from passing through the surface. • **N = mg cosθ** — on a horizontal surface N = mg; on an incline N = mg cosθ. • 💡 Wrong-option analysis: Applied Force: an applied force is externally applied by a person or agent, not the surface's own response; Gravitational Force: gravity pulls the book downward; it does not push it upward; Frictional Force: friction acts horizontally (parallel to surface) when there is relative motion, not vertically.

Correct Answer: D. Third Law

• **Third Law** = When a bullet is fired, the gun exerts a forward force on the bullet (action); the bullet exerts an equal and opposite backward force on the gun (reaction), causing recoil. • **action–reaction (Third Law)** — momentum is also conserved: bullet's forward momentum = gun's backward momentum. • 💡 Wrong-option analysis: First Law: first law is about inertia and constant velocity — not about action-reaction; Law of Inertia: this is another name for Newton's First Law, not Third; Second Law: second law (F = ma) gives force magnitude but doesn't explain the recoil direction mechanism.

Correct Answer: A. Inertial frame

• **Inertial frame** = Newton's laws hold without modification only in inertial reference frames — frames that move at constant velocity (no acceleration). • **inertial = non-accelerating** — pseudo-forces (like the Coriolis force) must be added in non-inertial frames. • 💡 Wrong-option analysis: Non-inertial frame: Newton's laws require correction (pseudo-forces) in non-inertial frames; Accelerating frame: an accelerating frame is non-inertial — Newton's laws don't directly apply; Rotating frame: a rotating frame is also non-inertial due to centrifugal and Coriolis effects.

Correct Answer: D. Force

• **Force** = Newton's Second Law in its general form: F = dp/dt; force equals the rate of change of linear momentum. • **F = dp/dt = ma** — when mass is constant, dp/dt = m dv/dt = ma, giving the familiar form. • 💡 Wrong-option analysis: Impulse: impulse = F × t (change in momentum), not the rate of change; Work: work = F × s (displacement), not rate of momentum change; Power: power = dW/dt (rate of work done), not rate of momentum change.

Correct Answer: C. To increase the time of impact

• **To increase the time of impact** = By pulling hands back, the cricketer extends the time Δt over which the ball's momentum reaches zero, reducing the average impact force (F = Δp/Δt). • **F = Δp/Δt** — same momentum change over longer time → smaller average force on hands. • 💡 Wrong-option analysis: To increase the force: the opposite — increasing time reduces force; To decrease the time of impact: decreasing time would increase force and risk injury; To decrease the momentum: the ball's momentum must reach zero regardless — the total change is fixed.

Correct Answer: C. Inertia of Direction

• **Inertia of Direction** = Inertia of direction is the resistance of a moving body to any change in its direction of motion without an external force. • **tendency to continue in straight line** — water droplets flying off a spinning wet cloth demonstrate inertia of direction. • 💡 Wrong-option analysis: Inertia of Rest: this is the resistance to starting motion from rest; Momentum: momentum is mass × velocity — it quantifies motion but isn't the name for directional inertia; Velocity: velocity is a kinematic quantity describing speed and direction, not a property of resistance.

Correct Answer: A. 10 N

• **10 N** = Using F = ma: F = 5 kg × 2 m/s² = 10 N. • **F = ma = 5×2 = 10 N** — straightforward application of Newton's Second Law. • 💡 Wrong-option analysis: 3 N: 3 = 5+2 (wrong operation — addition instead of multiplication); 7 N: 7 = 5+2 or some other incorrect arithmetic; 2.5 N: error of dividing 5 by 2 instead of multiplying.

Correct Answer: C. Newton's Third Law

• **Newton's Third Law** = Rockets expel exhaust gases backward at high velocity (action); the equal and opposite reaction force propels the rocket forward. • **action–reaction in vacuum** — unlike aircraft, rockets carry their own propellant and work even in the vacuum of space. • 💡 Wrong-option analysis: Archimedes' Principle: this relates to buoyancy of objects in fluids, not rocket propulsion; Conservation of Mass: this is a chemistry/physics principle but does not directly explain thrust generation; Pascal's Law: Pascal's law relates to pressure in confined fluids — unrelated to rocket propulsion.

Correct Answer: D. Accelerating

• **Accelerating** = The coin is tossed upward with the person's horizontal velocity; if the train accelerates forward, the train (and person) move ahead while the coin retains the original lower velocity and lands behind. • **inertia of the coin** — the coin follows a parabolic path in the ground frame but falls behind in the train's accelerating frame. • 💡 Wrong-option analysis: Moving with uniform speed: at uniform speed the coin would land back in the thrower's hand; Moving along a circular curve: a curve causes the coin to land to the side, not behind; Decelerating: if decelerating, the coin would land in front of the passenger, not behind.