A window washer pushes his scrub brush up a vertical window at constant speed by

Question

A window washer pushes his scrub brush up a vertical window at constant speed by applying a force F? as shown in the figure (Figure 1) . The brush weighs 12.8 N and the coefficient of kinetic friction is ?k=0.140 .

Part A

Calculate the magnitude of the force F? .

Express your answer in newtons to three significant figures. (N)

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Part B

Calculate the normal force exerted by the window on the brush.

Express your answer in newtons to three significant figures. (N)

Explanation / Answer

the normal force acting exerted by window on the brush = F*co53.1

frictional force acting on the brush = uk*N = uk*F*cos53.1 ( in down ward direction )

gravitational force acting down wards = Fg = m*g

verrtical component of force = F*sin53.1

net force Fnet = F*sin53.1 - m*g - uk*F*cos53.1

speed of the brush is constant , a = 0

from netons law Fnet = m*a = 0

therefore

F*sin53.1 - m*g - uk*F*cos53.1 = 0

F*sin53.1 - 12.8 - 0.14*F*cos53.1 = 0

F = 17.8 N   <<-----------answer

+++++++++++++++

normal force exerted by window = N = F*cos53.1 = 10.7 N <<-----answer

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