Friction

Mediumphysics

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

10 N

15 N

20 N

5 N

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Correct Answer:

20 N

An object starts moving when the applied external force $F$ exceeds the maximum value of static friction (limiting friction $f_{ma x}$ ).

Normal Force ( $N$ ): For a block on a horizontal surface, $N = m g$ . $N = 5 kg \times 10 m/s^{2} = 50 N$ .
Limiting Friction ( $f_{ma x}$ ): $f_{ma x} = μ_{s} N = 0.4 \times 50 N = 20 N$ . To start motion, the applied force must be at least equal to $20 N$ . If $F < 20 N$ , static friction will exactly cancel the applied force, and the block remains at rest.

This medium difficulty physics question is from the chapter laws of motion, covering the topic of friction. It appeared in the 2025 exam.

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