To determine which statement is incorrect, let's analyze each option:$\\$• Option 1: Frictional force opposes the relative motion.$\\$This statement is correct. Friction always acts in the opposite direction to the relative motion or intended motion between two surfaces.$\\$• Option 2: Limiting value of static friction is directly proportional to normal reaction.$\\$This statement is correct. The limiting static friction $(f_s)$ is given by $f_s = \mu_s \cdot N, \\$where $\mu_s$ is the coefficient of static friction and $N$ is the normal reaction force.$\\$• Option 3: Rolling friction is smaller than sliding friction.$\\$This statement is correct. Rolling friction is generally much smaller than sliding friction, which is why wheels are used to reduce friction.$\\$• Option 4: Coefficient of sliding friction has dimensions of length.$\\$This statement is incorrect. The coefficient of sliding friction is a dimensionless quantity.$\\$Therefore, the incorrect statement is Option 4.