MOMENTS

489

moment will be 4 ins. X3 lbs.=12 ineh-lbs. or 1 foot4b. The 1 Ib.
weight will thus be balanced by the 3 Ibs. weight and we have what
is called a system of Parallel Forces. The two forces of 1 Ib. and 3 Ibs.
acting downwards at W and W1 respectively are balanced by the
single, but equal and opposite force, acting upwards at F. The nail
at F is supporting a weight of 4 Ibs. neglecting the weight of the ruler.
The system is in equilibrium, and all such systems must be so when
the sum of the moments on one side of the fulcrum is equal to the sum
of the moments on the other side no matter how many weights and
distances there may be. The centre of gravity of the system is at the
fulcrum F and may be defined as the single force which is equal but
opposite to the resultant of the given forces.

w

T

__________________F| W

till It I I I i 1 I * j

I

Fig. 2.

Moment=power to turn =armx weight.

Example.—If a weight (W) of 10 Ibs. is suspended 3 feet from the
fulcrum of a freely rotating rod, where must a 3 Ibs. weight (TF1) be
suspended to regain the equilibrium of the systeml

Weight Tf1 X FW- = weight W X FW
Blbs.xFW1 =10 Ibs.x3ft.
3 F TP=30 foot-pounds .-. F 1^=10 ft.

Ans.—Place the 3 Ibs. weight 10 feet on the opposite side of fulcrum
to the 10 Ibs. weight.

When the ruler is supported with its centre of gravity over the
fulcrum the principle is the sam$ and the 1 Ib. weight 12 inches to the
right of F will balance the 3 Ibs. weight 4 inches to the left of F. We
have here a Lever, the simplest of machines. There are three kinds