To calculate the force exerted while holding a 10 kg book against a wall at a 30° angle, the friction between the book and the wall is crucial. The force applied must counteract both the weight of the book and any horizontal forces due to the angle of application. If the wall is considered frictionless, the book would not stay in place, indicating that friction is necessary for stability. The calculation involves resolving the forces into components and applying Newton's laws. Ultimately, understanding the role of friction is essential for accurately determining the force applied.