In GTA V, physics is a large factor in almost every part of the game. One example is when driving cars in GTA, you can reach some fairly impressive speeds. If you get into a car and start driving immediately, your character doesn't put on a seat belt which is a recipe for disaster. If you crash while driving at an insane speed, you will fly through the windshield and out of the car. This is because of inertia. As everyone knows, inertia is a property of matter. It is a measure of a resistance to acceleration based off of mass. When driving at fast speeds, the car is pushing you at the speed. When the car suddenly stops, your body wants to keep moving forward due to its inertia. Because of the lack of a restraint, your character flies through the windshield and skids across the road. Should've buckled up.