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Example1:

A box is dragged across a floor by a 100N force directed 60^o above the horizontal. How much work does the force do in pulling the object 8m?

 F = 100N

θ = 60^o

S = 8m

W = (F Cos θ) S

=(100 Cos 60^o) 8

= 100x1/2x 8 = 400 J

Example 2:

A horizontal force F pulls a 10 kg carton across the floor at constant speed. If the coefficient of sliding friction between the carton and the floor is 0.30, how much work is done by F in moving the carton by 5m?

The carton moves with constant speed. Thus, the carton is in horizontal equilibrium.

 F = f = μN = μmg.

 Thus F = 0.3 x 10 x 9.8

= 29.4 N

 Therefore work done W = FS

=(29.4 Cos 0^o)%

=147 J