Solved: PHYSICS Question for NEET

NEET PHYSICSEasy

Which one of the following statements is incorrect?

Rolling friction is smaller than sliding friction.

The limiting value of static friction is directly proportional to the normal reaction.

Frictional force opposes the relative motion.

The coefficient of sliding friction has dimensions of length.

Rolling vs Sliding Friction: Rolling friction is significantly smaller than sliding friction because the area of contact is momentary and deformed, and there is no gross sliding of surfaces [NCERT Class 11, Physics Part I, Sec 5.9.2 Rolling Friction]. Thus, Option 1 is correct.
Limiting Static Friction: The limiting value of static friction $(f_s)_{max}$ is given by the relation $(f_s)_{max} = \mu_s N$ , where $\mu_s$ is the coefficient of static friction and $N$ is the normal reaction. This indicates direct proportionality [NCERT Class 11, Physics Part I, Sec 5.9.1 Friction]. Thus, Option 2 is correct.
Direction of Friction: Frictional force always opposes the relative motion (or impending relative motion) between two surfaces in contact [NCERT Class 11, Physics Part I, Sec 5.9.1 Friction]. Thus, Option 3 is correct.
Dimensions of Coefficient: The coefficient of friction ( $\mu$ ) is defined as the ratio of frictional force to the normal force ( $\mu = f/N$ ). Since it is a ratio of two forces, it is a dimensionless quantity and has no units. It does not have dimensions of length [NCERT Class 11, Physics Part I, Sec 5.9.1 Friction]. Thus, Option 4 is incorrect.

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