Passengers sitting in a stationary car experience a jerk when the car suddenly starts. This is due to:

When the car starts, the people sitting in it experience a jerk. This is because, while the car was stationary, the people were stationary with it. Hence, they were having an inertia of rest. When the car starts, the inertia of the car changes from that of rest to that of motion. But the inertia of the people still remains that of rest. Hence, they experience a jerk.