you put your book on the bus seat next to you. when the bus stops suddenly, the book slides forward off the seat. why?
a. a net force acted on it
b. no net force acted on it

Question

jamesj · Answer

Newton's Law: A body in motion or at rest remains in motion or at rest unless a force acts on it.

The book is in motion and continues in motion.  Hence ...

anonymous · Answer

a net force acted on it?

jamesj · Answer

A body _continues in motion_ if no net force acts on it.

The book continues in motion.  Hence ...

anonymous · Answer

a.net force acted on it

jamesj · Answer

No, it continues in motion precisely because NO net force acted on it.