BODIES SUPPORTED IN TWO POINTS. 



109 



ilancers, etc., are applications of this principle : their 

 sntre of gravity lies outside their mass and below 

 ieir point of support ; hence they assume easily the 



position of stable equilibrium. 



A body which is supported in two points can no 



longer move freely in all directions ; it can only 



rotate about the straight line joining those two points. 



If such a body rotates, every point in it describes a 



FIG. 78 (an. proj. | real size). 



circle except those points which are situated in the 

 straight line between the points of support ; these 

 points are at rest, they are in the same condition as the 

 points of support. Hence if the centre of 'gravity of a 

 body be in the straight line joining the two points of 

 support, the body will be in neutral equilibrium. Such is 

 the case with the wheel in the machine for falling bodies 

 (p. 48) ; the centre of gravity, although not supported 

 directly, is in the same condition as the points really 

 supported ; the wheel is therefore in equilibrium in all 

 itions. If we combine this with the fact, previously 

 established, that a body is in equilibrium if supported 

 at any point in the line of gravitation, it follows : that 

 a body supported in two points will be in equilibrium 

 when the line of gravitation has one point in common with 

 the line joining the two points of supports, or, when these 



pos 



