Pending Internet Computerized Adaptive Test (PiCAT) Practice Test

When a book slides down a wall at a constant speed, it is in a state of dynamic equilibrium, meaning that the forces acting on it are balanced. The two primary forces involved in this situation are gravity and friction.

Gravity acts downward on the book, pulling it toward the ground due to its weight. This force is calculated using the mass of the book and the acceleration due to gravity. On the other hand, friction comes into play between the wall and the surface of the book. Since the book is sliding down, the frictional force acts upward, opposing the downward motion caused by gravity.

At constant speed, the force of friction exactly balances the force of gravity. Without this balance, the book would accelerate either downward (if gravity exceeded friction) or upward (if friction exceeded gravity). Therefore, in this case, the correct pair of forces acting on the book is gravity and friction, as they interact to maintain that constant speed during the descent.