Short answer
A force is a push or pull measured in newtons. Impulse is force acting over time, measured in newton-seconds, and equals the resulting change in momentum. The same average force creates more impulse if it acts longer. 1 2
Force is an instantaneous interaction; impulse accounts for both force and how long it acts.
On this page
At a glance
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| Question | Force | Impulse |
|---|---|---|
| Question | How strong is the interaction now? | What accumulated effect over time? |
| SI unit | Newton | Newton-second |
| Connection | Acts over an interval | Area under force-versus-time curve |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Force. An interaction that can change an object’s motion, measured in newtons. 1
Impulse. The effect of force accumulated over a time interval, equal to the change in momentum. 2
Key differences
For constant force, impulse equals force multiplied by duration. For changing force, the time integral matters. A force can be large yet brief, while a smaller force over longer time can yield comparable impulse. 1 2
How to tell them apart
If the problem gives an instantaneous push or a force measurement, use force. If it asks about change in momentum over the collision or a force-time interval, use impulse. 1 2
Where they overlap
Impulse is built from force and cannot be understood without a specified interval. Newton’s laws relate force to motion; the impulse-momentum theorem summarizes its accumulated effect. 1 2
Edge cases
Collision forces usually vary with time, so “force times duration” uses average force unless the force truly stays constant. Equal impulses need not have equal peak forces. 1 2
Why the distinction exists
Engineers and physicists need to distinguish peak loading from total momentum change, especially in impacts and safety design. 1 2
Common misconceptions
Impulse is not a different kind of force. It has different units and includes duration. 1 2
Examples
Catching a ball with hands that move backward lengthens stopping time, reducing average force for roughly the same change in momentum. 1 2
Sources
Sources checked October 3, 2026.
- OpenStax — Newton’s Second Law of Motion. Force and newtons.
- OpenStax — Impulse. Impulse-momentum theorem.