Seat belts are always Important
Newtons Laws of motion
Newtons first law is an object either is at rest or moves at a constant velocity unless acted upon by an external force
Newtons second law is acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass
Newtons third law is When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body
This is unbalanced force because it is not opposed by an equal and opposite force operating directly against the force intended to cause a change in the object's state of motion or rest. The dummies fly out of the car because they dont have there seatbelt on.
Forces are balanced when the forces pushing an object in one direction are the same size as the forces acting in the opposite direction. The dummies stay in the car because of the seatbelt.
Why do you need a seatbelt ?
You need a seat belt because it can save your life when you crash your car