A.2.18—Impulse-momentum change

Impulse is external to the system

For a chosen system, the net external impulse equals the system’s change in total momentum:

$$\vec J_{ext}=\Delta\vec p_{system}$$

Same momentum change, different force

If an object must undergo the same $\Delta p$, increasing the stopping time reduces the average resultant force:

$$F_{avg}=\frac{\Delta p}{\Delta t}$$

Apply to safety systems

A flexible safety net, airbag or crumple zone extends the interaction time while producing the required momentum change, reducing the average force on the person or vehicle.

Common trap

Extending the stopping time does not make the momentum change disappear; it changes the rate at which that change occurs.