Inertia vs Momentum — Resistance to Change vs Quantity of Motion
Inertia
The tendency of an object to resist changes to its state of motion — staying still or cruising steadily unless a net force acts. Measured simply by mass; it needs no motion at all.
Unit: kilogram (kg)
Example: A parked truck is hard to push not because it's moving (it isn't) but because its huge mass gives it huge inertia.
Momentum
The quantity of motion an object carries — mass times velocity. A vector: direction matters. In a closed system the total momentum is conserved through collisions.
Unit: kg·m/s
Example: A 0.15 kg baseball at 40 m/s carries 6 kg·m/s of momentum — that's what the bat must reverse to send it back.
Key differences
| Aspect | Inertia | Momentum |
|---|---|---|
| What it is | Resistance to change in motion | Amount of motion itself |
| Needs motion? | No — a parked truck has inertia | Yes — zero velocity means zero momentum |
| Vector or scalar? | Scalar (just mass) | Vector (direction matters) |
| Conserved? | Not a conserved quantity | Conserved in isolated systems |
| Depends on | Mass only | Mass and velocity |
'The truck has a lot of momentum standing still' is wrong — that's inertia talking. Momentum is zero when velocity is zero. Inertia explains why motion changes slowly; momentum bookkeeps how much motion exists.