CHAPTER VI 



PARALLEL FORCES AND CENTRE OF GRAVITY 



Parallel Forces. We have seen that the earth exerts a down- 

 ward pull upon all objects on its surface, and that in consequence 

 of this all things fall to the ground if unsupported. It follows, 

 therefore, that everything which is supported above the earth's 

 surface is constantly being pulled downwards, even though it 

 does not fall. If a beam, for instance, is supported horizontally 

 by resting the ends upon two posts, each particle of it may be 

 regarded as being pulled earthwards by an attractive force. 

 The direction of the pull is everywhere towards the centre of 

 the earth, so for any one spot on the earth's surface we may 

 consider the attractive forces due to gravity to be parallel to one 

 another. 



EXPT. 59. Place the ends of a stiff lath or rod of uniform 

 thickness upon two letter balances, or support the rod by 

 hanging each end from a spring balance. Notice the weight 

 borne by each balance ; then weigh the rod, and so determine 

 the proportion of the weight supported at each end. 



This experiment represents on a small scale the case of a 

 beam referred to before, and by using the spring balances we 

 further see that the weight of a beam is equally divided between 

 the two supports. In other words, we find that the two upward 

 forces exerted by the balances are together equal to the down- 

 ward force represented by the weight of the beam, 



EXPT. 60. Using the same arrangement as before, notice 

 the reading shown by each balance when the lath is supported- 





