A $20\, kg$ block is initially at rest on a rough horizontal surface. A horizontal force of $75 \,N$ is required to set the block in motion. After it is in motion, a horizontal force of $60\, N$ is required to keep the block moving with constant speed. The coefficient of static friction is
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(a)Coefficient of friction ${\mu _s} = \frac{{{F_l}}}{R} = \frac{{75}}{{mg}} = \frac{{75}}{{20 \times 9.8}}$$ = 0.38$
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