State Newton's first law (the law of inertia), relate inertia to mass, and apply the law to objects at rest and moving at constant velocity, recognizing that balanced forces produce no change in motion.

What this topic is asking

Newton's first law is the conceptual foundation of dynamics, and the Regents tests it mostly through reasoning: recognizing that an object at rest or moving at constant velocity has zero net force, and that inertia, the tendency to resist a change in motion, is measured by mass. The Physical Setting/Physics course wants you to apply the law to everyday situations (a coasting car, a book on a table) and to keep the idea of inertia distinct from weight.

Newton's first law

The law overturns the everyday intuition that motion needs a continual push. In reality, a moving object slows down only because resistive forces (friction, air resistance) act on it; remove those, as on a near-frictionless air track, and it coasts at constant velocity indefinitely. A net force is needed only to change motion, not to maintain it.

Inertia and mass

A loaded truck is harder to get moving and harder to stop than an empty one, because its larger mass gives it more inertia. This is why mass is sometimes called "inertial mass". Importantly, inertia and mass are properties of the object itself, unchanged by where it is, unlike weight.

Balanced forces and constant velocity

When the forces on an object add to zero, the object is in equilibrium and its velocity does not change. There are two equilibrium cases the Regents uses:

• Static equilibrium: the object is at rest and stays at rest (a book on a table: weight down balanced by the normal force up).
• Dynamic equilibrium: the object moves at constant velocity (a car cruising at steady speed: driving force balanced by friction and air resistance).

In both cases the net force is zero, which is exactly Newton's first law. Recognizing "constant velocity" or "at rest" as the signal for a zero net force is the key exam skill.

Mass versus weight

A frequent Regents distinction: mass is the amount of matter and the measure of inertia (kilograms, a scalar that is the same everywhere), while weight is the gravitational force on the object ($F_g = mg$, in newtons, a vector that varies with $g$). An astronaut has the same mass on the Moon as on Earth but weighs less, because the Moon's $g$ is smaller. Confusing the two is a classic error.

Reference Tables note

Newton's first law is a stated principle, not an equation, so it does not appear in the Reference Tables equation list. The related equation $F_g = mg$ (weight) is printed, and you use it to keep mass and weight distinct. The first law is really the special case of Newton's second law, $F_{net} = ma$, with $a = 0$, which is treated in Newton's second law.

Try this

Q1. State Newton's first law in your own words. [2 points]

• Cue. An object stays at rest or keeps moving at constant velocity unless a net (unbalanced) force acts on it.

Q2. State what property of an object measures its inertia. [1 point]

• Cue. Mass.