Q.

What is the maximum value of the force F such that the block shown in the arrangement does not move             [2003]

1 20 N  
2 10 N  
3 12 N  
4 15 N  

Ans.

(1)

Since the block is not moving forward for the maximum force F applied, therefore

Fcos60°=f=μN

(Horizontal direction)

For vertical equilibrium of the block,

N=mg+Fsin60°

 Fcos60°=μN=μ(Fsin60°+mg)

F=μmgcos60°-μsin60°=123×3×1012-123×32=514=20 N