If the momentum of a car is reduced by half, assuming mass did not change, what must also have been reduced by half?

Velocity is halved

Explanation:

Momentum is contingent on mass and velocity. The quantity of motion in a body is called the momentum of the body.

  Momentum  = mass x velocity

Now, if mass does not change and momentum is reduced by half, it suggests that the velocity was reduced by half.

Velocity is directly related to momentum.

in the analysis of systems that involve fluid flow, we frequently encounter the combination of properties u and pv.