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JAMB Physics 1987 Question 11

Question 11

Friction

The coefficient of static friction between a 40 kg crate and concrete surface is 0.25. Find the magnitude of the minimum force needed to keep the crate stationary on the concrete base inclined at 45° to the horizontal. [g = 10 m s−2]

  1. A.400 N
  2. B.300 N
  3. C.283 N
  4. D.212 N
Show the answer and explanation

Answer: D (212 N)

Explanation

Down the slope: mg sin 45° = 283 N. Friction can supply up to μmg cos 45° = 0.25 × 283 ≈ 71 N. The extra force needed is 283 − 71 ≈ 212 N.