If ${\mu _s}$,${\mu _k}$,and ${\mu _r}$ are coefficients of static friction,sliding (kinetic) friction,and rolling friction respectively,then:

$A$ body of mass $4\, kg$ is at rest on a horizontal table. The coefficient of friction between the body and the table is $0.2$. $A$ horizontal force of $6\, N$ is applied on the body. The acceleration of the body is .......... $m/s^2$.

$A$ wooden block of mass $10\,kg$ is placed on a rough horizontal surface. $A$ force of $49\,N$ is required to pull it. Find the coefficient of friction and the angle of friction.

Two masses $A$ and $B$ of $10\, kg$ and $5\, kg$ respectively are connected with a string passing over a frictionless pulley fixed at the corner of a table as shown. The coefficient of static friction of $A$ with the table is $0.2$. The minimum mass of $C$ that may be placed on $A$ to prevent it from moving is ........ $kg$.

The blocks $A$ and $B$ are arranged as shown in the figure. The pulley is frictionless. The mass of $A$ is $10 \, kg$. The coefficient of friction of $A$ with the horizontal surface is $0.20$. The minimum mass of $B$ to start the motion will be...... $kg$.

What is friction? What is impending motion?