Question

A 1.4 kg block slides across a rough surface such that it slows down with an acceleration of 1.25 m/s(squared). What is the coefficient of kinetic friction between the block and the surface?
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Answer 1

F = ma 

where 

F = frictional force 
m = mass of the block = 1.4 kg (given) 
a = acceleration of the block = 1.25 m/sec^2 (given) 

Substituting values, 

F = (1.4)(1.25) 

F = 1.75 N 

By definition, 

F = (mu)(Normal force) 

where 

mu = coefficient of friction 
Normal force = mg = 1.4*9.8 = 13.72 

Again, substituting appropriate values, 

1.75 = mu(13.72) 

mu = 0.128