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.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.