What is the phenomenon called when a stopped car suddenly moves forward and the people in the car lean backward?

The correct answer, which is inertia, refers to the tendency of an object to resist changes in its state of motion. In the scenario presented, when the car that has been stopped suddenly moves forward, the people inside the car experience a force due to acceleration. However, their bodies initially tend to remain in the state of rest due to inertia. As the car accelerates forward, the people are thrown backward relative to the car's movement because they are not experiencing the same acceleration immediately. This illustrates Newton’s first law of motion, which states that an object at rest will stay at rest unless acted upon by a net external force.

Momentum, while related to motion and involves mass and velocity, does not specifically explain the inclination of the passengers leaning backward during the acceleration of the car. Acceleration refers to the rate of change of velocity, which is essential in this scenario but does not address the reaction of the people in the car directly. Force relates to the interaction that causes acceleration but does not explain why the occupants feel the backward lean. Thus, inertia provides the clearest understanding of the movement and reaction of the passengers in this context.