Final answer:
When starting an automatic transmission in neutral, the car rocks in the opposite sense of the engine's rotation due to the conservation of angular momentum. However, the angular momentum is not conserved for long.
Step-by-step explanation:
When you start an automatic transmission in neutral and start the engine, the car rocks in the opposite sense of the engine's rotation due to the conservation of angular momentum. Angular momentum is the product of an object's moment of inertia and its angular velocity. While the engine is stationary, the wheels are not, and their angular momentum will be opposite to that of the engine, causing the car to rock.
However, the angular momentum of the car is not conserved for long because of the presence of external torques, such as friction and air resistance. These torques act on the car's rotating components, causing a change in angular momentum and ultimately bringing the car to a stop. Therefore, the car's rocking motion will eventually cease after a few seconds.