Which of the following is self-adjusting in nature?

Impending relative motion is opposed by which type of friction?

$A$ block of mass $M$ is being pulled along a rough horizontal surface. The coefficient of friction between the block and the surface is $\mu$. If another block of mass $M/2$ is placed on the block and it is again pulled on the surface,the coefficient of friction between the block and the surface will be

$A$ body of weight $50 \,N$ is placed on a horizontal surface as shown in the figure. The minimum force required to move the body is $28.28 \,N$. The frictional force and the normal reaction are respectively.

What is the maximum acceleration of a train such that a $50 \ kg$ box lying on its floor remains stationary (in $m \ s^{-2}$)? (Given: Coefficient of static friction between the box and the train's floor is $0.3$ and $g = 10 \ m \ s^{-2}$)

Calculate the maximum acceleration (in $m s^{-2}$) of a moving car so that a body lying on the floor of the car remains stationary. The coefficient of static friction between the body and the floor is $0.15$ $(g = 10 m s^{-2})$.