Suppose that you are standing on a train accelerating at 0.20g. What minimum coefficient of static friction must exist between your feet and the floor if you are not to slide.

Friction force and train accelerating force are opposite in direction and equal in magnitude. Therefore m x a = u x m x g

As the mass is same, hence both are cancel out and remaining equation is

a = u x g

It’s given that a = 0.20g, so

0.20g = u x g

Cancel "g" on both sides;

0.20 = u

Thus the coefficient of static friction = u = 0.20