Chapter 5. Forces and Motion

Describing motion. Speed = distance / time. Velocity includes direction. Acceleration = change in velocity / time. Near Earth’s surface, free-falling objects accelerate at about 9.8 m/s² (ignoring air resistance) [8][9].

Newton’s three laws of motion

Inertia: an object keeps its state of motion unless acted on by a net external force.

F = ma: net force equals mass times acceleration.

Action and reaction: when one object exerts a force on another, the second exerts an equal and opposite force on the first.

Common forces: gravity (weight W = mg), friction, normal force, tension and air resistance. The SI unit of force is the newton (N).

Gravity. Every mass attracts every other mass. Newton’s law of universal gravitation says the force is proportional to the product of the masses and inversely proportional to the square of the distance between them [8].

Momentum. p = mv. In the absence of external forces, total momentum is conserved.

Pressure. P = F/A, measured in pascals (N/m²).

Review: What net force accelerates a 10 kg object at 3 m/s²? (Answer: 30 N.)