Final answer:
When you start the engine with the transmission in neutral, the car rocks in the opposite direction due to the conservation of angular momentum. However, the angular momentum is not conserved for long and gradually decreases due to factors like friction and air resistance.
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. Angular momentum is a property of a rotating object and is conserved when no external torque acts on it. In this case, the engine's rotation creates an angular momentum, which is transferred to the car through the transmission. However, since the car is in neutral, there is no torque to counteract the angular momentum, causing the car to rock in the opposite direction.
The angular momentum of the car is not conserved for long. It gradually decreases as the rocking motion dissipates due to factors like friction and air resistance. The damping effect of these forces causes the car's angular momentum to decrease over time until it reaches equilibrium.