Newton's First Law of Motion: Inertia
Newton's First Law describes what happens when forces are balanced (or absent): nothing changes. An object at rest stays at rest, and an object in motion keeps moving at the same speed and in the same direction. This only changes when an unbalanced (net) force acts on the object.
Understanding this law is the foundation for everything else in this unit. It reframes how you think about motion: objects don't need a force to keep moving. They need a force to change how they're moving.
Newton's First Law of Motion: Inertia
Newton's first law of motion
The law 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 a net (unbalanced) force.
This is also called the law of inertia. Two key ideas follow from it:
- Objects at rest remain at rest unless a net force acts on them. A book sitting on a table won't slide off on its own. It stays put until something pushes it, pulls it, or otherwise exerts a force on it.
- Objects in motion continue at constant velocity (same speed, same direction) unless a net force acts on them. A ball rolling on a perfectly smooth surface would roll forever at the same speed. In reality, forces like friction and air resistance act on it, which is why it eventually slows down and stops.
That second point trips people up. Your everyday experience says "things stop moving on their own," but they don't. They stop because friction and air resistance are forces acting on them. In a frictionless environment (like deep space), a moving object would keep going indefinitely.
Effects of friction on motion
Friction is the force that opposes relative motion between two surfaces in contact. It always acts in the direction opposite to the motion (or attempted motion) of an object.
There are two main types:
- Static friction prevents an object from starting to move. If you push gently on a heavy box and it doesn't budge, static friction is matching your applied force.
- Kinetic friction opposes the motion of an object that's already moving. Once that box starts sliding, kinetic friction acts against the slide.
Friction connects directly to Newton's First Law because it's an unbalanced external force. A sliding box on a rough floor slows down and eventually stops, not because "motion naturally dies out," but because kinetic friction is continuously acting against the box's motion. Without friction, the box would slide forever at constant velocity.
Mass and inertia relationship
Mass is the amount of matter in an object, measured in kilograms (kg) in the SI system. It's an intrinsic property, meaning it doesn't change based on where you are. Your mass is the same on Earth, on the Moon, or floating in space.
Inertia is an object's resistance to any change in its motion, whether that's speeding up, slowing down, or changing direction. The relationship between mass and inertia is straightforward:
- Inertia is directly proportional to mass.
- More mass means more inertia, which means a greater force is needed to change the object's motion.
Think about pushing a shopping cart versus pushing a car. The car has far more mass, so it has far more inertia, and you'd need a much larger force to get it moving (or to stop it once it's rolling).
Mass is typically measured using a balance, which compares an unknown mass against standard known masses. Inertia itself isn't measured directly; you infer it from an object's mass and how it responds to forces.
Forces, Equilibrium, and Motion
A force is any interaction that, when unopposed, changes the motion of an object. Forces have both magnitude and direction (they're vectors).
Equilibrium is the condition where all forces on an object cancel out, meaning the net force is zero. When an object is in equilibrium, Newton's First Law tells you exactly what happens: its motion doesn't change. If it was at rest, it stays at rest. If it was moving at constant velocity, it keeps moving at that same velocity.
This is worth emphasizing: equilibrium does not mean "nothing is moving." It means "nothing is changing." A car cruising at a steady 60 km/h on a straight road is in equilibrium because the driving force and friction/air resistance balance out.
A few related terms to keep straight:
- Acceleration is the rate of change of velocity. It only occurs when there is a net force on an object. No net force means zero acceleration.
- Momentum is the product of mass and velocity (). Objects with more momentum are harder to stop, which connects back to the idea of inertia.
Newton's First Law, along with his Second and Third Laws, forms the foundation of classical mechanics, the framework for analyzing motion and forces throughout this course.