The property of matter that keeps your body in motion when the car comes to a halt is inertia.

Here's why:

* Inertia is the tendency of an object to resist changes in its motion. This means an object at rest wants to stay at rest, and an object in motion wants to stay in motion at the same speed and direction.

* When the car suddenly stops, your body, due to inertia, wants to continue moving forward at the same speed the car was traveling. This is why you feel pushed forward against the seatbelt.

Let me know if you have any more questions!