34 



JUNIOR GRADE SCIENCE 



To find the centre of gravity of a quadrilateral by construction, 

 the plan is to divide it into two triangles by drawing a diagonal. By 

 the method just described the centre of gravity of each triangle is 

 found, and the points so obtained are joined. The centre of gravity 

 of the quadrilateral lies on this line. Repeat the process drawing the 

 other diagonal. Join the centres of gravity of the second pair of 

 triangles, the centre of gravity of the quadrilateral lies on this line. 

 Hence, it is situated at the point of intersection of this line and the 

 first one obtained in the same way. 



Centres of gravity of other bodies. The method of drawing lines 

 across the surface of a thin plate is not practicable in the case of bodies 

 such as blocks or skeleton solids having three dimensions. The 

 experiments described in 14 ii. explain two methods which can be 

 employed usefully in cases of this kind. The ingenuity of the student 

 will provide other equally suitable plans for particular cases. 



15. EQUILIBRIUM. 



i. Conditions of equilibrium. Place upon a square edged table or board 

 one of the cardboard figures of which you have found the centre of gravity. 

 Gradually slide the figure near the edge until it would just topple over ; 

 keeping it in this position, draw a line along the under side of the cardboard 



STABLE 



NEUTRAL 



UNSTABLE 



FIG. 26. Relative positions of centre of gravity and point of support for stable, 

 neutral, and unstable equilibrium. 



where the edge of the table touches it. Then place the cardboard in another 

 position and again mark where the edge of the table touches it when it would 

 just topple over. The intersection of these lines is the centre of gravity, 

 and it will be noticed that the cardboard would just topple over when the 

 centre of gravity falls outside the edge of the table. 



ii. Stable, neutral, and unstable equilibrium. Procure an oblong strip of 

 wood or cardboard (Fig. 26). Support the strip as at A by a long pin 

 pushed through it; it is then in stable equilibrium, for the slightest turn 

 either to right or left raises the centre of gravity. When supported as at 



