Tricky pulley problem
Level
2
In the given diagram, with what force must the man pull the rope to hold the plank in position? Weight of the man is 60kgf . Neglect the mass of plank, rope and pulley
20kgf
60kgf
15kgf
30kgf
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1 solution
The conditions of equilibrium
F net = 0
Man on the plank as a system
T + N - mg = 0................................1
For plank as a system
3T - N = 0 ....................2
Substituting T from eq 1 in eq 2
we get 3 mg - 3 N - N = 0
3 mg = 4N
3/4 mg = N
now substitute N in eq 1
T = mg - 3/4mg
Mag of Weight = 60
T= 60 - 45
T= 15
Thus force applied is 15 kgf