Final answer:
The car rocks in the opposite direction of the engine's rotation due to conservation of angular momentum, which is a temporary effect disrupted by external forces such as friction and the car's suspension.
Step-by-step explanation:
When you start the engine of your car with the transmission in neutral, the car rocks in the opposite sense of the engine's rotation due to the conservation of angular momentum. The law states that the angular momentum of a system remains constant if no external torques act on it. In this case, when the engine starts and the crankshaft begins to rotate, it has a certain amount of angular momentum. To conserve this, the car body reacts by rotating slightly in the opposite direction.
Is the angular momentum of the car conserved for long? The answer is no. The effect is temporary because as soon as other forces such as friction and the car's suspension system come into play, they exert external torques on the system, and the car settles back into a stable state, thereby breaking the conservation of angular momentum within a few seconds.