Impulse measures the total effect of a force applied over a span of time: J = F × t, in newton-seconds. Its real power comes from the impulse–momentum theorem, which says that impulse equals the change in momentum, J = Δp = mΔv. That link means a force's effect on motion depends on how long it acts, not just how hard it pushes — and it explains why spreading an impact over more time, as airbags and crumple zones do, slashes the peak force for the same overall change in momentum.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the impulse–momentum theorem J = Ft = Δp.
The impulse equations
Multiply force by the time over which it acts to get impulse in newton-seconds. Because that equals the change in momentum, you can also read impulse straight off a before-and-after velocity change: J = mΔv. Dividing a known impulse by the object's mass returns the velocity change it produces. The newton-second and the kilogram-metre-per-second are the same unit, a direct consequence of the theorem.
Worked example — a push and its effect
Scenario: A constant 10 N force acts on a 4 kg object for 2 seconds. What impulse is delivered, and how much does the object's velocity change?
The impulse is 20 N·s, which changes the 4 kg object's velocity by 5 m/s. The same 20 N·s could come from a 20 N force over 1 s or a 4 N force over 5 s — different forces, identical momentum change. That trade-off is exactly what safety engineering exploits: stretch a crash from a few milliseconds to a couple of tenths of a second and the impulse stays fixed while the peak force, and the injury, falls sharply.
Frequently Asked Questions
J = F × t. e.g. 10 N × 2 s = 20 N·s. It equals the momentum change mΔv.
Impulse = change in momentum: J = Δp = mΔv. So 20 N·s on 4 kg gives Δv = 5 m/s.
Newton-seconds (N·s), identical to kg·m/s because impulse equals momentum change.
They lengthen the stopping time t, so the fixed impulse is delivered with a far smaller peak force.
Force (N) is instantaneous; impulse (N·s) is force × time. Duration matters as much as strength.