Forces and Newton's Laws: every key term you need (+ practice quiz)

More AP Physics 1 guides

• Kinematics
• Work, Energy and Power
• Momentum and Collisions
• Circular Motion and Gravitation
• Simple Harmonic Motion

Newton's First Law

Object at rest stays at rest; object in motion stays in motion unless external force applied. Force required for acceleration (change in velocity).

Inertia

Resistance to acceleration; measured by mass. Larger mass = harder to accelerate. Inertia not force; it's property of matter.

Newton's Second Law

F_net = ma. Net force equals mass times acceleration. Vector equation: same direction as net force.

Mass vs Weight

Mass: amount of matter (kg), constant. Weight: gravitational force (N), varies by location. W = mg.

Newton's Third Law

Action-reaction: for every force, equal and opposite reaction. Forces act on DIFFERENT objects. Same magnitude, opposite direction.

Free Body Diagrams

Sketch object with all forces acting on it. Arrows show magnitude/direction. Essential for solving dynamics problems.

Normal Force

Force perpendicular to surface, prevents penetration. On flat surface: N = mg (if no vertical acceleration). N ≠ mg on inclines/accelerating surfaces.

Friction Forces

Kinetic (sliding): f_k = μ_k·N. Static (no sliding): f_s ≤ μ_s·N. μ_s > μ_k usually. Friction opposes motion/tendency.

Inclined Planes

Resolve weight: parallel component mg sin θ (down slope), perpendicular mg cos θ (into surface). Normal force N = mg cos θ.

Tension

Force in rope/string; always pulls along string direction. Ideal rope: massless, inextensible. Same tension throughout if massless.

Pulleys

Ideal pulley: massless, frictionless. Changes direction of tension. If pulley moves, analyze separately.

Atwood Machine

Two masses connected by rope over pulley. Equations: (m₁ - m₂)g = (m₁ + m₂)a, T = 2m₁m₂g/(m₁+m₂).

Connected Objects

When objects move together with acceleration, treat as system: F_net = (total mass)·a. Then analyze individual object for internal forces.

Unit 2 Summary

Newton's laws: F=ma governs motion. Free body diagrams identify forces. Normal, friction, tension are common forces in problems.

Static vs Kinetic Friction Threshold

Static friction adjusts up to a maximum μ_s N; motion begins when the applied force exceeds this. Once sliding, friction drops to μ_k N, which is why a box lurches when it starts to move.

Apparent Weight in an Elevator

A scale reads the normal force, not mg. Accelerating upward: N = m(g + a); accelerating downward: N = m(g − a); free fall: N = 0 (weightlessness).

Normal Force Is Not Always mg

On an incline N = mg cosθ; with an applied force pushing down or pulling up at an angle, N changes accordingly. Always solve the perpendicular direction of Newton's second law for N.

Angle of Repose

A block on an incline just begins to slip when tanθ = μ_s. The result is independent of mass, so a heavier block does not stick better.

Systems Approach

For connected objects moving together, treat them as one system: a = F_external,net / m_total. Then isolate a single object to find internal forces like tension.

Tension in a Massless String

Tension is the same at every point of a massless, frictionless-pulley string. If the pulley has mass or friction, tensions on the two sides differ.

Third-Law Pairs Never Cancel

Action-reaction forces act on different objects, so they can never cancel in a single free-body diagram. Weight and normal force on a resting book are NOT a third-law pair.

Pulling vs Pushing at an Angle

Pulling up at angle θ reduces the normal force (N = mg − F sinθ) and hence friction; pushing down at an angle increases both. This is why pulling a sled is easier.

Frictional Force Direction

Kinetic friction opposes relative sliding between surfaces; static friction opposes impending motion and can point in the direction of the object's motion (a car's drive wheels, a box on an accelerating truck bed).

Non-Uniform Rope or Chain

In a rope with mass, tension is greatest at the end that is pulling the load and decreases toward the free end because each segment must accelerate only the mass beyond it.

Inertial Reference Frames

Newton's laws hold in frames that are not accelerating. In an accelerating car, objects appear to be pushed backward, but no real force does this; it is the car's frame that accelerates.

Equilibrium Conditions

Translational equilibrium requires ΣF = 0 in every direction; the object may be at rest or moving at constant velocity. Constant velocity does not require a net forward force.

Two-Body Contact Force

When a force F pushes block A into block B on a frictionless surface, the contact force on B is F·m_B/(m_A + m_B), the fraction of F needed to accelerate B alone.

Frictionless Incline Acceleration

a = g sinθ regardless of mass. On a rough incline sliding down, a = g(sinθ − μ_k cosθ); sliding up, friction adds so a = g(sinθ + μ_k cosθ) opposing motion.