Newton's First Law and InertiaNewton's First Law and Inertia

APPHYSICS1 · UNIT2 : F O R C E A N D T R A N S L A T I O N A L D Y N A M I C S

Newton's First Law and Inertia

AP Physics 1 Guide 1

Goal: Explain why an object keeps its motion unless a net force acts, and connect inertia to mass. Study notes in original wording, organized around a Newton's Laws slide deck and standard AP Physics 1 content. Use g = 9.8 m/s² unless your teacher says to use 10.

Big Picture

Newton's First Law says an object's velocity does not change unless an unbalanced (net) force acts on it. Motion does not need a force to continue. A force is needed to change motion. Isaac Newton (1643-1727) was an English physicist and mathematician whose three laws of motion and law of universal gravitation form the foundation of classical mechanics.

The Law

An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net (unbalanced) external force.

Constant velocity means constant speed in a straight line.

Net force is the vector sum of all forces: F_net = ΣF.

If ΣF = 0 then a = 0. If ΣF ≠ 0 the object accelerates (speeds up, slows down, or changes direction). Inertia and Mass

Inertia is an object's tendency to resist changes in its motion.

Mass (measured in kilograms) is the amount of matter in an object and is the measure of its inertia. More mass means more inertia, so it is harder to speed up, slow down, or turn.

Mass is the same everywhere in the universe. Weight is not (see Guide 2).

Inertia is not a force. It is a property of matter.

Equilibrium

Type Condition Example

Static equilibrium At rest and ΣF = 0 A book on a table

Dynamic equilibrium Moving at constant velocity and ΣF = 0 A car cruising at constant speed on a straight, level road Forces can be acting and still be balanced. "No net force" does not mean "no forces."

Motion at constant speed in a circle is not equilibrium, because the direction of the velocity is changing, so there must be a net force.

Everyday Examples

Seat belt: when a car stops suddenly, your body tends to keep moving forward at the old velocity until the belt applies a force.

Objects sliding to a stop on Earth: friction and drag are the unbalanced forces. In deep space with no forces, a moving object would keep its velocity.

Tablecloth trick: a heavy dish has large inertia, so a quick pull on the cloth changes its motion very little. Ball in an accelerating car: the ball tends to stay at its previous velocity, so relative to the car it appears to move backward.

Reasoning Patterns for Questions

If the problem says Then

Constant velocity Net force is zero; forces in each direction balance

At rest Net force is zero

Speeding up or slowing down Net force is not zero and points in the direction of the change in velocity Pushed at a steady speed on a rough floor The push equals the friction force, not zero friction Check Yourself

Q: A 2 kg cart rolls at a constant 3 m/s on a level track. A string pulls it forward with 4 N. What is the friction force? A: 4 N backward, so the net force is zero.

Q: A wrench is thrown in deep space with no forces acting. What happens? A: It keeps moving at the same constant velocity.

Q: Is a net force required to keep a puck moving at constant velocity on frictionless ice? A: No. Key Terms and Actors

Term / Actor What it is and why it matters

Newton's First Law (law of

inertia)

Velocity stays constant when net force is zero.

Inertia Resistance to changes in motion.

Mass Measure of inertia, in kg; same everywhere.

Net force Vector sum of all forces on an object.

Equilibrium Net force is zero; velocity is constant (including zero).

Constant velocity Constant speed in a straight line.

Making This Essay-Ready

Common misconception: "If it's moving, there must be a force pushing it forward." Only a change in motion requires a net force.

AP tip: Many questions ask which forces balance. Write ΣF_x = 0 and ΣF_y = 0 separately. AP Physics 1 Guide 1 · AP Physics 1 study notes