An object will remain at rest unless acted upon by an external force. In this picture, inertia is remaining unchanged with a balanced force until the car is moving it will be at an unbalanced force with a constant velocity.
the acceleration of an object is dependent on two variables- the net force being applied and the mass of the object. In this picture, the two drivers are not accelerating at the same speed because the mass of the sports car allows the driver to move at a faster rate, while both vehicles are being applied with the same net force they do not match up in speed.
For every action, there is an equal and opposite reaction. In this picture, both vehicles are coming to a halt meaning both cars are pushed back causing the drivers to also be pulled back with the car. The breaks of the car are exerting the action force of stopping, with the reaction force of being pulled back with the car. Without seatbelts, this action would cause drivings to fly through the car window.