Posted by Sara on .
A dock worker loading crates on a ship finds
that a 25 kg crate, initially at rest on a hori
zontal surface, requires a 90 N horizontal force
to set it in motion. However, after the crate
is in motion, a horizontal force of 51 N is
required to keep it moving with a constant
speed.
The acceleration of gravity is 9.8 m/s2 .
Find the coefficient of static friction be
tween crate and floor.

physics 
drwls,
90 N = M * g * mu_s
Solve for the static friction coefficient, mu_s
M = 25 kg
g = 9.8 m/s^2
The 51 N force can be used to compute the kinetic friction coefficient, but they didn't ask for that.