Newton's Laws of Motion Flashcards

A 2 kg block rests on a frictionless surface inside an elevator. The elevator accelerates downward at 4.9 m/s². What is the normal force exerted on the block by the elevator floor? (g = 9.8 m/s²)

Answer: 9.8 N

Using Newton's Second Law with net force = ma: N - mg = m(-a), so N = m(g - a) = 2(9.8 - 4.9) = 2 × 4.9 = 9.8 N. The downward acceleration reduces the apparent weight but does not eliminate it — only free fall (a = g) would give N = 0.

Two ice skaters (mass 60 kg and 90 kg) push off each other from rest. The 60 kg skater moves at 3 m/s after the push. According to Newton's Third Law, which statement best describes the force interaction?

Answer: Both skaters experience equal magnitude forces, but the 60 kg skater accelerates more

Newton's Third Law states that action-reaction forces are always equal in magnitude and opposite in direction, regardless of mass. Both skaters receive the same impulse (force × time). Because F = ma, the smaller mass (60 kg) undergoes greater acceleration: a = F/m. The 90 kg skater moves at 2 m/s (by conservation of momentum: 60×3 = 90×v).

A spacecraft traveling at constant velocity in deep space (far from any gravitational field) fires its thrusters briefly and then shuts them off. Which of Newton's Laws primarily explains why the spacecraft continues at its new constant velocity indefinitely?

Answer: Newton's First Law — an object in motion remains in motion unless acted upon by a net external force

Once the thrusters are off, there is no net external force in deep space (no gravity, no air resistance). Newton's First Law (the Law of Inertia) dictates that the spacecraft maintains its new constant velocity indefinitely. The Second Law describes what happens during the thrust; the Third Law explains how thrust is generated, but inertia explains the continued motion.

A 10 kg crate is pushed across a rough floor with a horizontal applied force of 50 N. The crate accelerates at 2 m/s². What is the coefficient of kinetic friction between the crate and the floor? (g = 10 m/s²)

Answer: 0.3

Using Newton's Second Law: F_net = F_applied - F_friction = ma → 50 - f_k = 10(2) = 20 N, so f_k = 30 N. The normal force N = mg = 10 × 10 = 100 N. Coefficient of kinetic friction μ_k = f_k / N = 30 / 100 = 0.3.

A horse pulls a cart with force F. According to Newton's Third Law, the cart pulls back on the horse with an equal and opposite force. A student argues this means the cart can never move. What is the fatal flaw in this reasoning?

Answer: The paired forces act on different objects, so they cannot cancel; the cart accelerates if the horse's pull on it exceeds the cart's friction with the ground

Newton's Third Law pairs always act on different objects — the horse pulls the cart forward, and the cart pulls the horse backward. To determine if the cart moves, you apply Newton's Second Law to the cart alone: F_horse_on_cart - F_friction_on_cart = m_cart × a. As long as the horse's pull exceeds ground friction on the cart, the cart accelerates. The reaction force on the horse is irrelevant to the cart's motion.

Object A (mass 4 kg) and Object B (mass 1 kg) are dropped simultaneously from the same height in a vacuum. Ignoring air resistance, which statement is true regarding the gravitational force on each object and their accelerations?

Answer: Object A experiences 4 times the gravitational force but the same acceleration as Object B

Gravitational force F = mg, so Object A (4 kg) experiences F = 4g = ~39.2 N and Object B (1 kg) experiences F = g = ~9.8 N — four times the difference in force. However, by Newton's Second Law, a = F/m = mg/m = g for both. The greater mass requires proportionally greater force to produce the same acceleration. Both objects fall with identical acceleration g ≈ 9.8 m/s², a key insight Galileo demonstrated.