The direction of static friction relative to the intended motion is:

• A. In the same direction as the intended motion.
• B. Perpendicular to the surface.
• C. Zero until motion begins.
• D. In the opposite direction to the intended motion.

Answer: (D)

Static friction always acts to prevent slipping by opposing the motion that would occur without it. If you push an object along a surface, the frictional force responds in the opposite direction to that intended motion. It can rise to a maximum value to keep the object at rest, so initially it can cancel your push and keep acceleration zero. Only when the push exceeds the maximum static friction does motion begin and kinetic friction takes over, still pointing opposite the actual motion. This is why the direction is opposite the intended motion, not the same, perpendicular, or zero except in a special no-tendency-to-move situation.