50 Domestic Science 



power of turning the rod about the fulcrum. For 

 instance, if one penny be placed on one arm 20 cm. 

 from the centre, it will be found to balance two placed 

 10 cm. from the centre. That is to say, the turning 

 power of each of these latter pennies is only one-half 

 that of the single penny situated twice as far from the 

 fulcrum. Again, if we put penny No. 1 40 cm. from 

 the middle of the rod, it will be found that we require 

 four pennies on the other arm at 10 cm. from the centre 

 to balance it, i.e. by doubling its distance from the 

 point about which the rod turns we have doubled its 

 turning-power. 



The turning-power of a force exerted in any similar 

 manner to that illustrated in the above experiment is 

 termed the " moment " of the force about the turning- 

 point, and the conclusion which may be drawn from 

 such results as the above is generally summarised as 

 follows : 



When the moments of two forces tending to turn 

 a body in opposite directions about a given fulcrum 

 are equal, the body remains at rest. 



This is the simplest case of a general principle of 

 much wider application, known as the " Principle of 

 Moments", and may be put in a brief mathematical 

 form thus : 



1st force x its distance from fulcrum 



= 2nd force x its distance from fulcrum. 



29. We are now ready to understand why an 

 ordinary chemical balance, or the less delicate instru- 

 ment known as a " pair of scales ", may be used to 

 determine the weight and hence the mass of a body. 

 Essentially a balance consists of a " beam ", able to 

 swing freely about its middle point, with pans attached 



