When a body is moving on a surface, the force of friction is called

Put a uniform meter scale horizontally on your extended index fingers with the left one at $0.00 cm$ and the right one at $90.00 cm$. When you attempt to move both the fingers slowly towards the center, initially only the left finger slips with respect to the scale and the right finger does not. After some distance, the left finger stops and the right one starts slipping. Then the right finger stops at a distance $x_R$ from the center ( $50.00 cm$ ) of the scale and the left one starts slipping again. This happens because of the difference in the frictional forces on the two fingers. If the coefficients of static and dynamic friction between the fingers and the scale are $0.40$ and $0.32$ , respectively, the value of $x_R($ in $cm )$ is. . . . . . . 

If ${\mu _s},\,{\mu _k}$ and ${\mu _r}$ are coefficients of static friction, sliding friction and rolling friction, then

What is kinetic friction ? What is rolling friction ?

Two bodies $A$ and $B$ of masses $5 kg$ and $10 kg$ in contact with each other rest on a table against a rigid wall. The coefficient of friction between the bodies and the table is $0.15$. A force of 200 $N$ is applied hortzontally to $A$. What are $(a)$ the reaction of the partition $(b)$ the action-reaction forces between $A$ and $B ?$ What happens when the wall is removed? Does the answer to $(b)$ change, when the bodies are in motion? Ignore the difference between $\mu_{ s }$ and $\mu_{ k }$

Explain dynamic friction. Write laws of kinetic friction. Define coefficient of kinetic friction.