Newton's First Law (Inertia)
An object stays at rest or in uniform motion unless acted on by a net force.
Formula
Variables
Example
A puck on frictionless ice keeps gliding at constant speed until something pushes it.
How it works
Newton’s First Law says that if the net force on an object is zero, its motion does not change: a stationary object stays at rest, and a moving object keeps the same speed and direction. This works because force is what causes acceleration, so when all forces balance out, the acceleration is zero and velocity stays constant. In real life, objects usually slow down only because forces like friction, air resistance, or gravity are acting on them.
When to use it
Use this law when analyzing motion in situations where the forces cancel out or are so small they can be ignored. It is especially useful for objects moving at constant speed in a straight line, or for checking whether an object should remain at rest.
Worked example, step by step
A hockey puck slides across very smooth ice with negligible friction. The forces on the puck are balanced, so net force = 0 N. Using Newton’s First Law: ΣF = 0 ⇒ v = constant. If the puck is moving at 3.5 m/s, it will keep moving at 3.5 m/s in the same direction. Final answer: the puck’s velocity stays 3.5 m/s because there is no net force.
Common mistakes to avoid
- Thinking an object needs a force to keep moving at constant speed
- Ignoring friction or air resistance when deciding whether the net force is zero
- Confusing constant velocity with constant speed even when direction changes
Did You Know?
Inertia is why passengers lurch forward when a car brakes — their bodies want to keep moving.
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More in Physics Laws & Theorems
View allNewton's Second Law
BasicNet force equals mass times acceleration (rate of change of momentum).
Newton's Third Law
BasicFor every action there is an equal and opposite reaction.
Conservation of Momentum
IntermediateTotal momentum of an isolated system stays constant.
Conservation of Energy
BasicEnergy cannot be created or destroyed, only transformed.
Archimedes' Principle
IntermediateBuoyant force equals the weight of fluid displaced.
Pascal's Law
IntermediatePressure applied to a confined fluid transmits equally in all directions.