Answer :
The coefficient of kinetic friction between the box and the floor is 0.2.
The direction of the kinetic friction force between the box and the floor is the opposite of the velocity, as depicted in the free body diagram on the attached picture.
The second Newton's law of motion:
∑ F = m. a
Let F1 be the person force and f be the kinetic friction force.
Since the box is just about to move, it means a = 0.
F1 - f = 0
f = F1 = 100 Newton
Let N be the normal force and w be the weight of the box.
In the vertical direction:
N - w = 0
N = w = 500 Newton.
It is known that:
f = μ . N
Where: μ = coefficient of kinetic friction.
Hence,
μ . N = 100
μ = 100/N = 100/500 = 0.2
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