If the mass of an object is doubled,what will be the effect on the coefficient of friction?

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 $g = 10 \ m \ s^{-2}$)

If the mass of block $A = 10\,kg$,the coefficient of static friction $\mu_s = 0.2$,and the coefficient of kinetic friction $\mu_k = 0.2$,find the mass of block $B$ required to start the motion.

$A$ force $F = mg$ is acting on a block of mass $m$ as shown in the figure. The pushing of the block along the surface is possible if:

The limiting value of static friction between two contact surfaces is ...........

In the figure shown,a horizontal force $F_1$ is applied on a block,but the block does not slide. Then,as the magnitude of the vertical force $F_2$ is increased from zero,the block begins to slide. Which of the following statements is correct?