Final answer:
To start a car with an automatic transmission, the gear selector should be in park or neutral. The car may rock due to conservation of angular momentum when started in neutral, but this is quickly dissipated by other forces.
Step-by-step explanation:
When starting a car with an automatic transmission, the gear selector lever must be in either park (P) or neutral (N). In the context of physics, particularly the principle of conservation of angular momentum, this is marginally relevant. Particularly, when an engine starts with the transmission in neutral, the car may exhibit a rocking motion opposite the engine's rotation due to the engine's turning affecting the car's stationary mass, showcasing Newton's third law of motion. However, the angular momentum of the car is not conserved for more than a few seconds as the car itself is not in a closed system and various forces, such as friction and those exerted by the engine mounts, quickly dissipate the motion.