Energy is not the only thing a moving body carries. Momentum is $p = mv$, read aloud p equals m v: $p$ is the momentum, $m$ is the mass in kilograms and $v$ is the velocity in metres per second. Its unit has no special name — it is simply kg·m/s, and you write it out.

Set it beside kinetic energy and the differences are the lesson. Momentum has no square and no half, so doubling the speed doubles it, not quadruples it. Momentum is a vector: it points where the velocity points, and a body going the other way carries NEGATIVE momentum, which is what makes collisions cancel so beautifully. And momentum survives collisions that energy does not — the coupling that turns half your kinetic energy into noise and heat leaves the momentum untouched.

What changes a momentum is an impulse: $J = F \, \Delta t$, J equals F delta-t, where $J$ is the impulse, $F$ is the AVERAGE force in newtons and $\Delta t$ — delta-t, delta meaning “change in” — is the contact time in seconds. Its unit is N·s, and here is the elegant part: N·s and kg·m/s are the same unit. They have to be, because the impulse IS the momentum change.

Now read the relation as a design instruction, because that is what it is. A vehicle arriving with a certain momentum must lose all of it — that quantity is fixed the moment it enters the barrier. Only $\Delta t$ is negotiable. Stretch the stop from a hundredth of a second to a tenth and the average force falls by ten. That is the crumple zone, the airbag, the arrester bed and the climbing rope, all of them selling the same thing: time.