Question

Choose the correct statements. The coefficient of static friction depends on:
A) the material of the bodies in contact.
B) quality of machining of the surfaces in contact.
C) presence of foreign matter between the surfaces.
D) area of the surfaces in contact.

Answer 1

Hint: The resistance faced by a body in static condition while it tends to move under the action of external force. This resistance is called the static frictional force. The ratio of limiting friction (maximum static friction) to normal reaction is called the coefficient of static friction. Using the above statements determines the factors that the coefficient of static friction depends on.

Complete step by step solution:
Friction is the opposing force that comes between two surfaces so as to impede the relative motion between them. Friction is regarded as an electromagnetic force. Friction can also be due to the cohesive or adhesive force among molecules at close proximities.
The resistance is faced by a body in static condition while it tends to move under the action of external force. This resistance is called the static frictional force. The ratio of limiting friction (maximum static friction) to normal reaction is called the coefficient of static friction. It is a unitless quantity.

The coefficient of static friction drastically changes when dust and impurities are present on the surface. The coefficient of static friction depends on the surface finish of the material, the nature of the material but is independent of the area of the body and mass of the body

Hence options $\left( A \right)\left( B \right)\left( C \right)$ are the correct options.

Note: The surfaces in contact will slip on each other without opposition in the absence of friction. This may be producing a dangerous motion. Polishing, using lubricants, using ball bearing, streamlining can reduce friction. Another effective way to reduce friction is to place a thin cushion of air maintained between the solid surfaces in relative motion.