Friction

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A 5 kg block rests (μ_s = 0.4). Minimum horizontal force to start motion? (g=10 m/s²)

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An object starts moving when the applied external force $F$ exceeds the maximum value of static friction (limiting friction $f_{max}$).
1. **Normal Force ($N$)**: For a block on a horizontal surface, $N = mg$.
   $N = 5 	ext{ kg} 	imes 10 	ext{ m/s}^2 = 50 	ext{ N}$.
2. **Limiting Friction ($f_{max}$)**:
   $f_{max} = \mu_s N = 0.4 	imes 50 	ext{ N} = 20 	ext{ N}$.
To start motion, the applied force must be at least equal to $20 	ext{ N}$. If $F < 20 	ext{ N}$, static friction will exactly cancel the applied force, and the block remains at rest.

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