When a car suddenly stops the objects in the back seat are thrown forward. this is due to?

Isaac newton's first law of motion says that any object which is moving will continue moving in the same direction and speed unless acted on by an outside force. This property of matter is sometimes called "inertia".

When your car is in motion, all the things in it are also in motion. When you hit the brakes, the brakes become the "outside force" that slows the car down, but the stuff inside your car has only a fairly minor force - the friction of the object and the seat of the car - to slow it down. So the object tends to continue to move in the same direction until it hits something, such as the back of the front seat.