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JAMB Physics 1989 Question 7

Question 7

Friction

A block of mass m is held in equilibrium against a vertical wall by a horizontal force. If the coefficient of friction between the block and the wall is μ, the minimum value of the horizontal force is

Diagram for this question
  1. A.μmg
  2. B.(1 − μ)mg
  3. C.(1 + μ)mg
  4. D.mg/μ
Show the answer and explanation

Answer: D (mg/μ)

Explanation

The horizontal force F is the normal reaction, so the wall can give an upward friction of at most μF. This must hold up the weight: μF = mg, so F = mg/μ.