Newton's First Law of Motion
Inertia & Equilibrium
Newton's First Law, also known as the Law of Inertia, states that an object at rest will stay at rest, and an object moving at a constant velocity will continue moving at that velocity, unless acted upon by a net external force.
This means objects resist changes to their state of motion. This property is called inertia, and it's directly related to the mass of the object—the more mass, the greater the inertia.
Understanding Inertia
Inertia explains why you feel pushed back into your seat when a car accelerates suddenly, or why a rolling ball eventually stops due to friction, which is an external force acting against its motion.
Historical Context
Before Newton, the dominant belief (from Aristotle) was that objects needed continuous force to keep moving. Newton revolutionized physics by showing that motion is natural and only changes when forces act on objects.
Equilibrium Explained
When the net force on an object is zero, it is said to be in equilibrium. This means:
- Static Equilibrium: Object is at rest and stays at rest.
- Dynamic Equilibrium: Object moves with constant velocity (no acceleration).
Mathematically, ∑F = 0 ⇒ constant velocity, where ∑F is the sum of all forces acting on the object.
Common Misconceptions
- Objects need a force to keep moving — actually, force is needed only to change motion, not maintain it.
- Heavier objects fall faster — Newton's laws show all objects accelerate equally (ignoring air resistance).
Summary Table
| Concept | Description | Example |
|---|---|---|
| Inertia | Resistance to change in motion, proportional to mass. | A passenger lurching forward when a car stops suddenly. |
| Equilibrium | Net force equals zero, no change in velocity. | A book resting on a table or a puck sliding at constant speed on ice. |
| Net Force | Sum of all forces acting on an object. | Friction, gravity, applied push or pull. |