Which of the following statements about the concept of solid friction is NOT true?

Analyze the question: We need the statement that is NOT true about solid (dry) friction. Four statements are true, and one is false.

Concept: Friction is a force that opposes the relative motion (or attempted motion) between two surfaces in contact. Key facts:

• It acts opposite to the direction of motion (or to the direction in which the object tends to move).
• It depends on the nature (roughness) of the surfaces and on the normal reaction, F = μR.
• It is independent of the area of contact (for solid friction).
• It causes wear and heat, and it reduces machine efficiency by wasting energy.

Checking each option:

• A. It always acts in the direction of motion. FALSE. Friction acts opposite to the direction of motion, not along it. This is the statement that is NOT true.
• B. It causes wear and tear in car tyres. True. Rubbing between tyre and road wears the tyre away.
• C. It depends on the nature of the surfaces in contact. True. The coefficient of friction μ depends on the materials and their roughness.
• D. It is independent of the area of the surfaces in contact. True. This is one of the laws of solid friction.
• E. It reduces the efficiency of the machine. True. Energy is lost as heat overcoming friction, so output is less than it should be.

Answer: A.

Tip: In "NOT true" questions, look for absolute words like "always". Here, remember that friction opposes motion. Friction can sometimes help motion (for example, it is what lets you walk forward), but even then it opposes the relative sliding between your foot and the ground, so "always in the direction of motion" is wrong.