1.27 Know and use the principle of moments for a simple system of parallel forces acting in one plane

For this, you need to know how to manipulate the equation (Moment = F x D). If the straight line is balanced, the moment on both sides: clockwise and anticlockwise, will be the same.

Eg A

mass 1                     .                              mass 2
                                /\

If mass 1 weighs 50 N and is 10m from the pivot and mass 2 weighs 200N and is 5m from the pivot, what is the moment caused by mass 1 and mass 2?

Moment 1 = F x D
= 50 x 10
= 500 NM                                    (Moments measured in newton meters; NM)

Moment 2 = F x D
= 200 x 5
= 1000 NM

Eg B

mass A                     .              mass B                mass C
                                 /\

If mass A weighs 100N and is 5m from the pivot, mass B is 2m from the pivot and weighs 50 N and mass C is 8m from the pivot and weighs 50N, what is the moment on the left side and the right side?

Moment L = F x D
= 100 x 5
= 500 NM

Moment R1 = F x D
= 50 x 2
= 100 NM

Moment R2 = F x D
= 50 x 8
= 400 NM

Moment R1 + R2 = 500 NM
                                                                                                                                                                   

In example B, the moment on the left side and the right side is equal.