Javier's notebook has fallen out of his bag and is on the floor of the bus. Before he can grab it, the bus accelerates forward and Javier's notebook slides backward. Why does his notebook move this way?

It follows Newton's first law of motion (law of ineria)

Explanation:

From Newton's first law of motion, a body at rest tends to maintain its initial state or constant motion in a straight line unless acted upon by external force. This is also known as the law of inertia. Considering that the notebook has fallen down and is on the bus floor, this then is at rest. When the bus accelerates, this notebook will obey the law of inertia by remaining at rest. While the bus starts to move, the book tries to remain at rest and that is why it seems to slide backwards.