A boy sits motionless partway down a playground slide. The force of friction balances the force of gravity. A girl hands the boy a book. Why does the boy still not move? A. Holding the book does not effectively increase the mass of the boy. B. Newton's first law of motion does not apply because the boy is at rest. C. The force of friction and the force due to gravity increase together. Friction and gravity are still balanced. D. The mass of the book cancels out part of the mass of the boy.

C.

The force of friction = coefficient of friction * normal force.

Adding the book to the boy increases the normal force and the component of the gravitational force directed down the slide. This in turn increases the force of friction as can be seen by the relationship from the above equation. For a stationary object, the force of static friction is always equal to the force applied (in this case, it is the component of the gravitational force directed down the slide). That means that so long as the boy is not moving and his mass increases, the frictional force is increasing also to balance the increased downward gravitation force directed down the slide.