13.
when all the forces acting upon an object each other, the object will not accelerate.

a. focus
b. balance
c. push against
d. slide past

B balance

if u have any doubts or more questions feel free to contact

B

Explanation:An object is said to be acted upon by an unbalanced force only when there is an individual force that is not being balanced by a force of equal magnitude and in the opposite direction.

The book is in motion and at the moment there is no one pushing it.

The force of gravity pulling downward and the force of the table pushing upwards on the book are of equal magnitude and opposite directions. As the book moves to one direction, friction acts to the opposite direction to slow the book down. There is an unbalanced force; and as such, the book changes its state of motion. The book is not at equilibrium and will accelerates.

this could be newton#'s law of cooling. the cooling curve is exponential

not too sure thinking all accelerations are pointed down