Which of the following is a self-adjusting force?

Explain: "Static friction force opposes impending motion".

The maximum force of friction is called:

$A$ $20\, kg$ block is initially at rest on a rough horizontal surface. $A$ horizontal force of $75\, N$ is required to set the block in motion. After it is in motion,a horizontal force of $60\, N$ is required to keep the block moving with constant speed. The coefficient of static friction is

As shown in the figure,a block of mass $10 \ kg$ lying on a horizontal surface is pulled by a force $F$ acting at an angle $30^{\circ}$ with the horizontal. For $\mu_{s} = 0.25$,the block will just start to move for the value of $F$: [Given $g = 10 \ ms^{-2}$] (in $N$)

$A$ block of $1\, kg$ is stopped against a wall by applying a force $F$ perpendicular to the wall. If $\mu = 0.2$,then the minimum value of $F$ will be ....... $N$.