A 25.0 kg block is initially at rest on a horizontal surface. A horizontal force of 75.0 N is required to set the block in motion, after which a horizontal force of 60.0 N is required to keep the block moving with constant speed. Find

(a) the coefficient of static friction.

(b) the coefficient of kinetic friction between the block and the surface.

(a) 0.31

(b) 0.245

Explanation:

(a)

F' = μ'mg ... Equation 1

Where F' = Horizontal Force required to set the block in motion, μ' = coefficient of static friction, m = mass of the block, g = acceleration due to gravity.

make μ' the subject of the equation above

μ' = F'/mg ... Equation 2

Given: F' = 75 N, m = 25 kg

constant: g = 9.8 m/s²

Substitute these values into equation 2

μ' = 75 / (25*9.8)

μ' = 75/245

μ' = 0.31.

(b) Similarly,

F = μmg ... Equation 3

Where F = Horizontal force that is required to keep the block moving with constant speed, μ = coefficient of kinetic friction.

make μ the subject of the equation

μ = F/mg ... Equation 4

Given: F = 60 N, m = 25 kg, g = 9.8 m/s²

Substitute these values into equation 4

μ = 60 / (25*9.8)

μ = 60/245

μ = 0.245