Which one of the following statements is incorrect?

Question

Accepted Answer

1. **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.
2. **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.
3. **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.
4. **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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