Newton's Laws of Motion Flashcards

A 2 kg block rests on a frictionless surface and is connected via a massless string over a frictionless pulley to a 3 kg hanging mass. What is the acceleration of the system?

Answer: 5.88 m/s²

Using Newton's Second Law on the system: net force = (3 kg)(9.8 m/s²) = 29.4 N, total mass = 2 + 3 = 5 kg, so a = 29.4 / 5 = 5.88 m/s². Only the hanging mass provides the driving force; both masses accelerate together.

A rocket in deep space (no gravity, no air resistance) fires its engine for 10 seconds, producing a constant thrust of 5000 N. If the rocket's mass decreases from 1000 kg to 900 kg during this burn, which statement best describes the rocket's acceleration during the burn?

Answer: Increasing, because the mass is decreasing

By Newton's Second Law, a = F/m. With constant thrust (F = 5000 N) and decreasing mass, the acceleration must increase over time. At the start, a = 5000/1000 = 5 m/s²; at the end, a = 5000/900 ≈ 5.56 m/s². The acceleration is not constant — it grows as mass is expelled.

Two ice skaters, Alex (80 kg) and Blake (50 kg), stand facing each other and push off simultaneously. Alex moves away at 2.5 m/s. What is Blake's speed, and which law directly explains this outcome?

Answer: 4 m/s — Newton's Third Law

By conservation of momentum (rooted in Newton's Third Law — equal and opposite forces act for equal times): 0 = (80)(2.5) − (50)(v), so v = 200/50 = 4 m/s. The Third Law dictates that the forces are equal and opposite, and combined with the Second Law, yields different accelerations for different masses.

A book sits on a table. A student claims the reaction force to the book's weight (Earth pulling the book down) is the table pushing the book up. Why is this student INCORRECT?

Answer: The reaction force is the book pulling Earth upward, not the table's normal force

Newton's Third Law pairs must act on different objects and involve the same interaction type. The weight of the book is Earth pulling the book (gravitational interaction). The reaction pair is the book pulling Earth upward — same force type, reversed objects. The table's normal force is a contact (electromagnetic) interaction, a separate force pair entirely.

A 10 kg crate is pushed across a floor with a horizontal applied force of 40 N. The kinetic friction force is 15 N. Suddenly the applied force is removed. According to Newton's First Law, what will the crate do immediately after the force is removed?

Answer: Decelerate due to the net unbalanced friction force

Newton's First Law states an object moves at constant velocity only when net force is zero. After the applied force is removed, friction (15 N) still acts — a net unbalanced force. The crate therefore decelerates (a = −15/10 = −1.5 m/s²) rather than continuing at constant velocity. Constant velocity requires zero net force, not merely zero applied force.

A 5 kg object hangs from two angled ropes: one pulls at 60° above horizontal with tension T₁, and a second pulls at 30° above horizontal with tension T₂. The system is in static equilibrium. Which condition, derived from Newton's Second Law, MUST hold true?

Answer: T₁ sin60° + T₂ sin30° = 49 N and T₁ cos60° = T₂ cos30°

For equilibrium, net force = 0 in every direction. Vertical: T₁ sin60° + T₂ sin30° = mg = (5)(9.8) = 49 N. Horizontal: T₁ cos60° = T₂ cos30° (the horizontal components must cancel). Option B states both conditions correctly. The other options confuse component directions or incorrectly sum magnitudes.