a balanced force is being applied. a frictional force is being exerted. an unbalanced force is being applied. a gravitational force is being exerted.
I gave my students this multiple choice question: If a book sitting on a flat table starts to move horizontally, it is most likely moving because

Only one answer is correct among these 4 choices:
- a balanced force is being applied.
- a frictional force is being exerted.
- an unbalanced force is being applied.
- a gravitational force is being exerted.

Could you tell me which one is correct?

an unbalanced force is being applied.