ISRO Mechanical & Aerospace 2017 - Question 50

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Topic: Engineering Mechanics

A block (4x3 m size) of 20 kg mass rests on a flat horizontal surface. If the coefficient of friction is 1/4, the force 'F' which will cause the block to move is approximately

Figure for question 50, ISRO Mechanical & Aerospace 2017

  • A. 20 kgf
  • B. 10 kgf
  • C. 5 kgf
  • D. 8.7 kgf

Answer: C. 5 kgf

Explanation: The normal force N = mg - Fsin(30). Friction force f = μN = 0.25(200 - 0.5F). For motion, Fcos(30) > f. So, 0.866F > 50 - 0.125F. This gives 0.991F > 50, so F > 50.45 N, which is approximately 5.04 kgf.

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