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Question:

Which one of the following statements is incorrect?

Rolling friction is smaller than sliding friction

Coefficient of sliding friction has dimensions of length

Limiting value of static friction is directly proportional to normal reaction

Frictional force opposes the relative motion

Solution:

Sliding friction is given by F = μN or μ = F/N. Force and Normal reaction both are given in Newton, so the μ is dimensionless. Therefore, statement B is wrong.