Calculate the sliding friction coefficient if the frictional force of a 6 g throw on a horizontal surface is 0.04N.

m = 6 g = 0.006 kg.
Ftr = 0.04 N.
g = 9.8 m / s ^ 2.
μ -?
The sliding friction force Ftr is determined by the formula: Ftr = μ * N, where μ is the coefficient of friction of rubbing surfaces, N is the reaction force of the support (the force with which the body is pressed against the surface).
The support reaction force N is determined by the formula: N = m * g.
Ftr = μ * m * g.
μ = Ftr / m * g.
μ = 0.04 N / 0.006 kg * 9.8 m / s ^ 2 = 0.68.
Answer: the coefficient of sliding friction of the bar on the surface is μ = 0.68.