Which of the following is a self adjusting force?

A block of mass $40 \,kg$ slides over a surface, when a mass of $4 \,kg$ is suspended through an inextensible massless string passing over frictionless pulley as shown below. The coefficient of kinetic friction between the surface and block is $0.02$. The acceleration of block is ............ $ms ^{-2}$ (Given $g =10 \,ms ^{-2}$.)

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 $\mu_{ k }$

Consider a car moving on a straight road with a speed of $100\, m/s$. The distance at which car can be stopped, is ........ $m$. $[\mu_k = 0.5]$

Consider a car moving along a straight horizontal road with a speed of $72\, km/h$. If the coefficient of kinetic friction between the tyres and the road is $0.5,$ the shortest distance in which the car can be stopped is ........ $m$ .$[g = 10\,m{s^{ - 2}}]$

When a body is lying on a rough inclined plane and does not move, the force of friction