A book is on top of a table. Earth's gravity exerts a constant force of 15N on the book. Why does the book not move as a result of this force?

The table exerts an equal upward force or 15 N on the book. The net force is zero.

the force of gravity is balanced by the force of friction

The book does not move as a result of the force because there is another force acting on it in the opposite direction. This force is called the normal force, and it is exerted by the table on the book.

To understand why the book stays in place, we need to consider Newton's laws of motion. Newton's first law states that an object at rest will stay at rest, and an object in motion will stay in motion with the same speed and direction, unless acted upon by an unbalanced force. In this case, the book is at rest, so according to Newton's first law, it will stay at rest unless there is an unbalanced force acting on it.

The gravitational force pulling the book downward is 15N, but the normal force exerted by the table upward is also 15N. These two forces are equal in magnitude but opposite in direction, which means they cancel each other out.

When the book is placed on the table, the force of gravity pulls it downwards, and simultaneously, the table pushes the book upwards with an equal force. The two forces balance each other out, resulting in a net force of zero. Since the book experiences no net force, it remains stationary on the table.

In summary, the book does not move as a result of Earth's gravity because there is an equal and opposite force called the normal force exerted by the table, which cancels out the gravitational force and keeps the book in place.