Newton’s First Law of Motion

Have you noticed how a sleeping pet stays put until nudged? Or how your body leans forward when a car suddenly brakes? These everyday observations reflect Newton’s First Law of Motion:

A body at rest remains at rest, and a body in motion continues in motion at constant velocity unless acted on by a net external force.

This principle means that motion doesn’t spontaneously begin or end—it changes only if something (a force) causes it to change. If no net external force is applied, objects preserve their state of motion. This is sometimes called the law of inertia.

Let’s think biologically. Blood flowing through your arteries keeps moving unless resistance (from narrowing vessels or plaque) slows it down. Air continues into the lungs unless obstructed. Cells and organs follow these same mechanical rules of motion.

Friction and External Forces

In daily life, objects often slow down or stop due to friction, a common external force. Consider a puck on an air hockey table: with the air off, it quickly comes to rest due to friction. With the air on, it glides much farther—nearly frictionless motion. If friction were completely removed, the puck would coast forever in a straight line.

This is the essence of Newton’s First Law: motion changes only with external cause.

For biology students, this concept shows up everywhere. Muscles apply forces to initiate or stop motion. Neurons fire to trigger muscle contractions. Even microscopic processes like vesicle transport within a cell rely on balanced or unbalanced forces.

Cause and Effect: A Fundamental Scientific Shift

Galileo and Newton helped shift scientific thinking away from abstract philosophy and toward empirical observation. Newton’s First Law formalized the idea that all changes in motion have physical causes—something essential in all sciences, including physiology, neuroscience, and biomechanics.

Mass and Inertia

The tendency of an object to resist changes in motion is called inertia. This is why pushing a heavy medical cart takes more effort than pushing an empty one. The more mass something has, the more inertia it has.

Mass is a measure of the amount of matter in an object. It’s also a measure of how much inertia the object has.

Mass is different from weight. Mass doesn’t change based on location—it’s the same whether you’re on Earth or the Moon. Weight, however, depends on gravity.

Check Your Understanding

Which has more mass: a kilogram of cotton balls or a kilogram of gold?
→ They have the same mass: 1 kilogram. But gold is denser, so it takes up less space.