Question

The friction forces existing between the surfaces in relative motion is known as

• A. Static friction
• B. Angle of Repose
• C. Kinetic friction
• D. Rolling resistance

Hint:

Static friction acts before motion starts, while kinetic friction acts when surfaces are in relative motion.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
This question asks for the specific term used to describe the frictional force that acts between two surfaces when they are moving relative to each other. This requires knowledge of the different types of friction.
Step 2: Detailed Explanation:
Let's define the types of friction listed:
(A) Static friction: This is the frictional force that opposes the initiation of motion between two surfaces in contact. It acts when the surfaces are at rest relative to each other. Its magnitude can vary from zero up to a maximum value ($f_{s,max} = \mu_s N$).
(C) Kinetic friction: This is the frictional force that opposes the motion between two surfaces that are already sliding or moving relative to each other. Its magnitude is typically constant for a given pair of surfaces and is given by $f_k = \mu_k N$. The key phrase in the question is "in relative motion," which directly corresponds to the definition of kinetic friction.
(B) Angle of Repose: This is not a force. It is the maximum angle of a slope at which a pile of granular material remains stable. It is related to static friction between the particles.
(D) Rolling resistance (or Rolling friction): This is the force that resists the motion when an object (like a wheel or a ball) rolls on a surface. While it is a type of friction that occurs during motion, the term "kinetic friction" is the general term for sliding friction between surfaces in relative motion.
Step 3: Final Answer:
The friction force that exists between surfaces in relative motion is known as kinetic friction. Therefore, option (C) is correct.