An object of mass 5.0kg rests on a rough incline at an angle of 20∘ to the horizontal. Your teacher states that the object remains at rest because the component of gravity down the slope is exactly balanced by static friction. A classmate presents the following free-body diagram:
• Arrow labeled mg straight downward.
• Arrow labeled N perpendicular to and away from the surface.
• Arrow labeled fs perpendicular to the surface and pointing upward away from the surface.
Assume the incline is the only surface in contact with the object and air resistance is negligible.
(a) Describe the conceptual error in the direction of the static friction force as shown in the diagram and explain how you know the direction is incorrect.
(b) Redraw the free-body diagram, showing all correct forces acting on the object and labeling each force clearly.
(c) Using components parallel and perpendicular to the incline, write the conditions that must be satisfied for the object to remain at rest. Explain how those conditions relate to your corrected diagram.