Definition Check – Friction Force and Direction of Motion A force acting opposite to the direction of motion (or impending motion) of a body is called the force of friction. Do you agree?

Introduction / Context:
Friction is a resistive contact interaction that opposes relative motion between surfaces. Correctly identifying its direction is crucial in free-body diagrams used for equilibrium and dynamics problems.

Given Data / Assumptions:

• Two solid surfaces in contact.
• Either actual relative motion (kinetic) or tendency to move (static).
• No lubrication effects altering the regime.

Concept / Approach:
Static friction acts to oppose the impending relative motion between surfaces. Kinetic friction acts to oppose the actual relative sliding velocity. Therefore friction acts opposite to the direction of relative motion (or its tendency) at the contact interface.

Step-by-Step Solution:
If the body slides to the right on a rough surface, kinetic friction acts to the left. If a force attempts to move a block rightward but it has not moved yet, static friction acts leftward to resist. Thus, in either case, friction opposes motion or impending motion.

Verification / Alternative check:
For rolling without slipping, friction direction depends on external torques but still acts to oppose the relative motion at the point of contact (which is instantaneously at rest for pure rolling). The concept remains consistent.

Why Other Options Are Wrong:
Disagree or restricted cases (only rolling/fluids/zero speed) misstate the general definition for dry contact friction.

Common Pitfalls:
Confusing the direction of friction in rolling problems; analyze relative motion at the contact point. Assuming friction always points opposite to the body’s center-of-mass velocity; the correct reference is relative motion at the interface.

Final Answer:
Agree