If a system will consist only one pulley then in that case we will have
[tex]F_{input} = F_{output}[/tex]
here we will say
[tex]F_{output} = 120 N[/tex]
As we know that we need to pull 120 N weight so here we need to apply same force on the other side
So here we can say that on the other side of pulley the applied force must be 120 N
