Question

Which of the following statements is correct about friction?

A) The coefficient of friction between a given pair of substances is largely independent of the area of contract between them

B) The friction force can never exceed the reaction force on the body from support of the surface.

C) Rolling friction is only slighting smaller than sliding friction

D) The main cause of friction is the irregularity of the surface in contact.

Answer 1

Hint: The ratio of force required to move two sliding objects over each other and the force pressing them together is defined as the coefficient of friction. The coefficient of friction only depends on the nature of the material.

Complete step-by-step answer:

Frictional force generally depends on only two factors: The nature of material, especially their surface texture and the pushing force of both bodies(While pushed together, more surface area comes into contact) $ F = \mu {F_N} $  (Coulomb's equation).

Here  $ {F_N} $  is the normal force whereas  $ \mu  $  is the coefficient of friction. Therefore we can say  $ \mu  $ (coefficient of friction is largely independent of area.

There are cases where frictional force exceeds the normal force (reaction force) i.e.,  $ \mu  $  greater than 1. For example silicon rubber can have  $ \mu  $  greater than one.

When an object is sliding over another surface the force experienced by the object is called sliding friction (also known as kinetic friction).When an object rolls over the surface , the friction experienced is rolling friction. There is a difference ratio of sliding to rolling friction. There are rolling friction cases where the coefficient of rolling friction is much smaller than sliding friction.

Example-Riding and sliding bicycle tyres is an example .

As mentioned earlier the main source for factors influencing friction is pushing force and nature of bodies.

Hence correct answer is option (A).

Note: Friction force in fluids is proportional to velocity for small values of velocities .As the velocity increases this proportionally breaks. For solid to solid friction ,frictional force is almost independent of relative velocity to an extent. As relative velocity decreases i.e. for very low relative velocity, friction usually goes up as there is more time available to stick. The further effect due to relative velocity in friction is complex and under scientific study.