What Is Newton's First Law?
Newton's First Law of Motion states: An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by an unbalanced force.
This is also called the Law of Inertia.
What Does That Actually Mean?
Imagine a hockey puck on ice. Once you hit it, it keeps sliding. It doesn't slow down much because there's very little friction. On Earth, things eventually stop because of friction and air resistance — but those are forces. Without forces, motion continues forever.
Similarly, a book on a table doesn't spontaneously start moving. It takes a force (like your hand pushing it) to make it move.
Inertia: The Resistance to Change
Inertia is the tendency of objects to resist changes in their motion. The more massive an object, the more inertia it has.
- A bowling ball is harder to push than a tennis ball
- A moving truck is harder to stop than a bicycle
- An oil tanker takes miles to stop even with engines reversed
Everyday Examples
- Seatbelts: When a car stops suddenly, your body wants to keep moving (inertia). Seatbelts provide the force to stop you.
- Tablecloth trick: Pull a tablecloth quickly, and dishes stay put. They have inertia and prefer to stay at rest.
- Spacecraft: In the vacuum of space, spacecraft coast forever after their engines shut off. No friction means no force to slow them.
Why It Matters
Newton's First Law seems obvious, but it was revolutionary. For millennia, people believed objects naturally slow down. Aristotle thought motion required continuous force.
Newton showed that motion is natural — it's changes in motion that require force. This insight underlies all of modern physics.
Common Misconceptions
Misconception: "Objects naturally stop moving."
Reality: Objects stop because of forces (friction, air resistance). In space, they'd move forever.
Misconception: "Heavier objects fall faster."
Reality: All objects accelerate at the same rate due to gravity (ignoring air resistance). Galileo proved this.
Learn More
This article is based on our audio course Physics Fundamentals. Listen to Episode 2: "Classical Mechanics: Newton's Laws of Motion" for the complete explanation.