Physics

The key insight is recognizing that "just sufficient to start moving" means you're at the threshold where applied force equals maximum static friction. Static friction isn't a fixed value—it grows with the normal force until it hits its limit, $f_{s,\max} = \mu_s N$. Since the block is on a horizontal table, the normal force equals the weight, so you can set the applied force equal to $\mu_s mg$ and solve for $\mu_s$.

The key insight is recognizing that "just sufficient to start moving" means you're at the threshold where applied force equals maximum static friction. Static friction isn't a fixed value—it grows with the normal force until it hits its limit, $f_{s,\max} = \mu_s N$. Since the block is on a horizontal table, the normal force equals the weight, so you can set the applied force equal to $\mu_s mg$ and solve for $\mu_s$.