The maximum possible force of static friction between two surfaces is given by:
F = μN
F = max friction force, μ = coefficient of static friction, N = normal force
The normal force N equals the object's weight:
N = mg
m = mass, g = gravitational acceleration
Make a substitution:
F = μmg
Given values:
F = 500N, m = 100kg, g = 9.8m/s²
Plug in and solve for μ:
500 = μ(100)(9.8)
μ = 0.51