Explain, in terms of impulse and momentum, how airbags help avoid injury in a car crash?

Explanation:

Impulse is the change in momentum.  It is also equal to the average force times the change in time.

J = Δp

F Δt = m Δv

The change in momentum is a fixed number; your mass, initial velocity, and final velocity are constant.

F Δt = k

So force and time are inversely proportional.  What this means is, we can either have a large force over a short period of time, or a small force over a long period of time.

Obviously, we want a small force over a long period of time.  Without airbags, your head would hit the steering wheel very quickly.  With airbags, your head has time to decelerate, so there's less force.