Final answer:
The car rocks in the opposite direction when the engine starts due to the conservation of angular momentum, which is only conserved briefly because real-world forces like friction act on the car.
Step-by-step explanation:
Conservation of Angular Momentum in Car Engines
When you start the engine of your car with the transmission in neutral, what you're observing is the conservation of angular momentum. This is a principle of physics that states that if no external net torque acts on a system, the total angular momentum of that system remains constant. In the case of the car engine, when the engine starts and begins to rotate, it has angular momentum. According to the conservation principle, the car must exhibit an equal and opposite angular momentum, which is seen as the car rocking in the opposite direction of the engine's rotation. This effect is brief because as soon as other forces (like friction and engine mount resistance) act on the car, they exert external torques that change the system's total angular momentum.
Therefore, the correct answer to why the car rocks in the opposite sense of the engine's rotation is due to conservation of angular momentum, and this conservation is only brief (for more than a few seconds) because real-world forces quickly begin to act on the car, altering its state of motion.