19) 700 N

This requires the knowledge of Moments (physics term). 

In order for the bar to be in equilibrium, Moments clockwise = Moments anticlockwise

Moments = F x perpendicular distance

Moments anticlockwise = 1.5 x 1000 = 1500 Nm

Moments clockwise = 100 x 4.5 = 450 Nm

Moments required = 1500 – 450 = 1050 Nm

Total length of the bar = 1.5 + 4.5 = 6 m

Since the weigh of the bar acts on the middle, the weight would act at (6 ÷ 2) = 3 m

Distance of weight from the pivot = 3 – 1.5 = 1.5m

Moments required = 1050 Nm

1050 = F x 1.5

F = 700 N

Hence weight = 700 N