Newton's Laws of Motion Flashcards

A 5 kg block rests on a frictionless surface. Two horizontal forces act on it simultaneously: 12 N to the right and 7 N to the left. What is the magnitude of the block's acceleration?

Answer: 1 m/s²

By Newton's Second Law, net force = 12 N − 7 N = 5 N to the right. Acceleration = F_net / m = 5 N / 5 kg = 1 m/s². The block's prior motion state does not affect how Newton's Second Law is applied — the net force determines acceleration regardless.

An astronaut in deep space (no gravity, no air) gives a 2 kg tool a push, exerting 10 N for 0.5 seconds. According to Newton's Third Law, what force does the tool exert on the astronaut during those 0.5 seconds?

Answer: 10 N opposite to the push

Newton's Third Law states that action and reaction forces are always equal in magnitude and opposite in direction, regardless of the masses involved. The tool exerts exactly 10 N back on the astronaut — mass difference only affects the resulting acceleration, not the force pair.

A book sits motionless on a table. A student claims the normal force from the table and the gravitational force on the book are a Newton's Third Law action-reaction pair. Why is this claim incorrect?

Answer: Because both forces act on the same object (the book), so they cannot be a Third Law pair

Newton's Third Law pairs always act on two DIFFERENT objects. Both the normal force (table on book) and gravity (Earth on book) act on the book itself — they are a balanced First Law pair, not a Third Law pair. The true Third Law counterpart to gravity on the book is the book pulling Earth upward; the counterpart to the normal force is the book pushing the table downward.

A 1000 kg car accelerates from rest at 2 m/s². After 4 seconds, the engine is cut and the car decelerates at 1 m/s² due to friction. What net force acts on the car during deceleration?

Answer: 1000 N backward

During deceleration, F_net = m × a = 1000 kg × (−1 m/s²) = −1000 N, meaning 1000 N directed backward (opposing motion). The earlier acceleration phase is irrelevant to the force during deceleration — only current mass and current acceleration matter.

Two ice skaters, A (mass 80 kg) and B (mass 40 kg), stand still and push off each other. Skater B moves away at 3 m/s. What is skater A's speed immediately after the push?

Answer: 1.5 m/s

By Newton's Third Law, the forces are equal and opposite, and the push duration is the same — so each skater receives equal and opposite impulses. Since impulse = change in momentum: 80 kg × v_A = 40 kg × 3 m/s → v_A = 120/80 = 1.5 m/s. This also follows from conservation of momentum (initial total = 0).

A passenger stands in an elevator holding a scale under their feet. The scale reads LESS than the passenger's true weight. Which of the following must be true according to Newton's Laws?

Answer: The elevator has a net downward acceleration

The scale reads the normal force on the passenger. If the reading is less than true weight (mg), then N < mg, meaning the net force is downward: mg − N = ma → a is directed downward. This occurs whenever the elevator accelerates downward (speeding up going down OR slowing down going up). Constant speed — in any direction — would give a reading equal to true weight.