Newton's First Law

Newton's First Law, the law of inertia, states that a body at rest tends to remain at rest unless acted upon by a net external force. When the bus starts, the floor and the passenger's feet are dragged forward by friction, but the upper body has no such immediate forward agent acting on it. Because of inertia, the upper body tries to maintain its original state of rest, so relative to the accelerating bus it appears to move backward. The option claiming the bus exerts a backward force is wrong because the only horizontal contact force, friction at the feet, actually acts forward. The friction-pushes-backward option misidentifies the direction of that force. The real-backward-gravity option is wrong because gravity always acts vertically downward and is unrelated to horizontal lurching. This is the classic NCERT illustration of inertia of rest used to introduce the First Law. As a plausibility check, the same reasoning predicts a forward lurch when the bus brakes, which matches common experience and confirms the inertia explanation.