Question

# A block A of mass 45 kg is placed on another block B of mass 123 kg. Now block B is displaced by external agent by 50 cm horizontally towards right. During the same time block A just reaches to the left end of block B. Initial and final positions are shown in figures. The work done on block A in ground frame is

A
-18 J
B
18 J
C
36 J
D
-36 J

Solution

## The correct option is C 18 JAs there is relative motion so kinetic friction will act on A in the direction of motion of B (or towards right).$$f=\mu mg=0.2\times 45\times 10=90 N$$$$S=50-30=20 cm=0.2 m$$$$\therefore W=fS=18 \ J$$Physics

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