The frame of reference in which Newton's laws are valid is called an inertial frame of reference. The object relative to this frame of reference continues to stay at rest or move with a constant velocity. Consider a car in motion. The passenger is moving along with the car relative to the Earth. When the car stops suddenly, the passenger experiences a push forward despite no net force acting on her. Although the passenger moves forward relative to the car, her velocity relative to the Earth is maintained. The inertia of the passenger tends to maintain her velocity relative to the Earth due to Newton's first law of motion. Here, the frame of reference attached to the Earth can be considered as an inertial frame of reference. Even though the Earth's velocity is not constant due to its rotation and revolution, it can be approximated as an inertial frame as its effects are negligible on objects' motion on Earth's surface.