Problem :
The coefficient of static friction, μ_{s}, between a given block of
weight 50N and a surface has a value of .75. A horizontal force is
applied to the block. How much force must be applied for the block to
move?
When exactly enough force is applied for the block to move, the
static frictional force will be equal to its maximum possible value.
Thus:
F_{s} = F_{s}^{max} = μ_{s}F_{N}
Because the block rests on a horizontal plane, the normal force is simply
equal to the weight of the block:
F_{N} = 50N. Thus
F_{s} = (50)(.75) = 37.5N.
Recall that the static frictional force always cancels exactly with the
force applied to the block. Thus the minimum necessary force to move the
block is simply
37.5N.