# You put your book on the bus seat next to you. When the bus suddenly stops, the book slides forward off the seat. Why?

(A) a net force acted on it
(B) No net force acted on it
(C) it remained at rest
(D) it did not move, but only seemed to

Toolbox:
• Newton's first law states that when viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.
Answer: no net force acted on it
The book was initially moving forward (because it was on a moving bus).
When the bus stopped, the book continued moving forward, which was its initial state of motion, and therefore it slid forward off the seat.