Question

Which one of the following statements is incorrect ?

• A. Coefficient of sliding friction has dimensions of length.
• B. Rolling friction is smaller than sliding friction.
• C. Frictional force opposes the relative motion.
• D. Limiting value of static friction is directly proportional to normal reaction.

Answer 1

The Correct Option is A

The question asks us to determine which statement is incorrect regarding friction. Let's analyze each option:

• Option 1: Coefficient of sliding friction has dimensions of length.
  Explanation: The coefficient of sliding friction (often denoted as \mu) is a dimensionless quantity. It is the ratio of the frictional force acting between two bodies and the normal force acting on them. Since both these forces are measured in Newtons, the coefficient is the ratio of two forces and therefore has no dimensions.
  This statement is incorrect.
• Option 2: Rolling friction is smaller than sliding friction.
  Explanation: In general, rolling friction is indeed smaller than sliding friction because rolling friction involves less surface area and less deformation of the surfaces in contact.
  This statement is correct.
• Option 3: Frictional force opposes the relative motion.
  Explanation: The primary role of friction is to resist or oppose the relative motion between two surfaces in contact. This is a fundamental property of frictional forces.
  This statement is correct.
• Option 4: Limiting value of static friction is directly proportional to normal reaction.
  Explanation: The limiting value of static friction (maximum static friction) is given by the formula: f_s = \mu_s N where \mu_s is the coefficient of static friction and N is the normal reaction. This shows direct proportionality.
  This statement is correct.

After analyzing all the options, the incorrect statement is: Coefficient of sliding friction has dimensions of length. Hence, this is the correct answer.