Question

Fill in the blanks-
Dynamic fluid friction is _________ the sliding friction of solids.
(A) Higher than
(B) Lower than
(C) Same as
(D) Higher than or equal to

Answer 1

Hint: Friction is defined as the force which opposes the relative motion of solids surfaces, fluid layers, and materials sliding against each other. Dynamic friction is lower than the sliding friction of solids.

Complete step-by-step answer:
Friction is defined as a force that opposes the relative motion of the material. Friction is caused due to surface roughness and deformations. Fluid friction is observed between fluid layers that are moving opposite to each other. This internal resistance to flow is called viscosity. There is a total of four types of friction:
Static friction
Sliding friction
Rolling friction
Fluid friction
In solids, we define two types of friction (1) static friction (2) kinetic friction. The fourth type of friction that is fluid friction occurs in gases and fluids.
Kinetic friction is also called sliding friction. The reason for kinetic friction in solids can be explained at the nanoscale by thermodynamics. The coefficient of static friction is more than that of dynamic fluid friction because static friction works between the two surfaces which are in contact just started a relative motion or in rest.

Hence, option (B) is the correct option.

Additional Information:
We have three friction defined in solids namely statics friction, kinetic or sliding friction, and rolling friction.
In fluids, viscosity acts as frictional force and it is called fluid friction.
Static friction is the friction in which objects are at rest. It is given by, here $f$ is friction force, is the coefficient of static friction and $N$ is a normal reaction to the surface. Maximum
 Frictional force before objects start sliding is $F = {\mu _s}N$ this is also called limiting friction.
Kinetic friction occurs when objects are moving relative to each other and give by where the ${\mu _k}$ coefficient of kinetic friction.

Note: In fluids, viscosity acts as friction.
Viscosity is defined as thickness. For example, water is less thick; it means its viscosity will be less. That in turn means fluid friction will be less in water.