It always acts opposite to the direction of motion.
Explanation
Friction is a resistive contact force that arises when two surfaces move or attempt to move across each other. It acts parallel to the surfaces in contact and opposes relative motion.Analysis of Options:
- It always acts in the direction of motion. is incorrect: Friction is a retarding force. It does not act in the direction of motion; instead, it works to reduce the relative velocity between the object and the surface.
- It always acts opposite to the direction of motion. is correct: For an object already in motion (involving kinetic friction), the frictional force always acts in a direction opposite to the instantaneous velocity of the object relative to the surface.
- It always acts perpendicular to the direction of motion. is incorrect: The force acting perpendicular to the surface of contact is the Normal Force, not friction. Friction acts tangentially to the surface.
- It is independent of the direction of motion. is incorrect: The direction of friction is not independent; it is intrinsically linked to the direction of motion because it must always oppose it.
Key Takeaway: Kinetic friction is a vector quantity that always acts tangentially to the contact surface and opposite to the direction of relative motion.