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

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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.

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.