Short answer
Momentum is mass times velocity. Impulse is force applied over time, or more generally the time integral of force, and equals the resulting change in momentum. They have equivalent units but answer different questions: what momentum is now, and how much it changed. 1 2
Momentum describes a state of motion; impulse describes how much that momentum changes during an interaction.
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At a glance
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| Question | Momentum | Impulse |
|---|---|---|
| Question | How much motion state? | How much change during interaction? |
| Simple formula | p = mv | J = FΔt for constant force |
| SI unit | kg·m/s | N·s, equivalent to kg·m/s |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Momentum. A vector quantity equal to an object’s mass times velocity in classical mechanics. 1
Impulse. The effect of force over a time interval, equal to the change in momentum. 2
Key differences
OpenStax derives impulse from the net force acting over a time interval and connects it to change in momentum. A large momentum need not imply a large impulse at that instant; an impulse is identified across an event. 1 2
How to tell them apart
If mass and velocity are given at one moment, calculate momentum. If force and contact time or before-and-after velocities are given, calculate impulse. Preserve direction because both quantities are vectors. 1 2
Where they overlap
Impulse changes momentum, and both share the same dimensions. Collisions, braking and launches are naturally described with both terms. 1 2
Edge cases
Force may vary throughout the interval; then impulse is the area under a force–time curve, not simply one force value times time. For a system, use net external impulse and define the system boundary. 1 2
Why the distinction exists
Separating state from change clarifies why increasing collision time can lower average force for the same momentum change. 1 2
Common misconceptions
Impulse is not a synonym for momentum or a sudden desire to act. A stationary object can receive an impulse and acquire momentum. 1 2
Examples
A moving baseball has momentum before being caught. The glove exerts an impulse that brings its momentum toward zero. 1 2
Sources
Sources checked October 3, 2026.
- OpenStax — Linear Momentum and Force. Linear momentum.
- OpenStax — Impulse. Impulse-momentum theorem.