Let w denote the weight of the sofa, n the magnitude of the normal force, and f the magnitude of the friction force. The sofa is in equilibrium, so by Newton's second law,
n - w = 0
125 N - f = 0
The sofa has a weight of
w = (132 kg) g = 1293.6 N
where g = 9.80 m/s² is the magnitude of the acceleration due to gravity. Since n = w, the normal force has the same magnitude.
The friction force is proportional to the normal force by a factor of the coefficient of static friction µ, such that
f = µ n
Our second equation tells us f = 125 N, so solve for µ :
125 N = µ (1293.6 N)
µ ≈ 0.0966
