A force on its own changes nothing until it lasts. Multiply it by how long it acts and you get the impulse: $J = F \, \Delta t$, read aloud J equals F delta-t — $\Delta$ is the Greek letter delta, said “delta”, and it always means “the change in”, so $\Delta t$ is the length of the contact in seconds. $J$ is the impulse and $F$ is the average force in newtons — average, because the real force during a bat's swing or a bumper's crush spikes and collapses in a shape nobody wants to integrate at 8 a.m.

Now look at impulse's unit: $\mathrm{N \cdot s}$, which is $\mathrm{kg \cdot m/s^2 \times s} = \mathrm{kg \cdot m/s}$ — momentum's unit exactly. That is not a coincidence, it is the next lesson's headline: an impulse IS a delivery of momentum. And it is why airbags exist. The momentum your body has to lose in a crash is fixed by your mass and the car's speed; the only thing an engineer can buy you is $\Delta t$. Stretch the stop from 5 ms to 100 ms and the same $J$ arrives with one-twentieth the force. Bend your knees when you land — you have been solving this equation since you were a toddler.