PARALLEL FORCES AND CENTRE OF GRAVITY 89 



CHIEF POINTS OF CHAPTER VI. 



Parallel Forces. The resultant of a number of parallel forces is 

 numerically equal to the sum of those which act in one direction, 

 less the sum of those which act in the opposite direction. In other 

 words the resultant of a system of parallel forces is equal in inar/ni- 

 ttifte to the algebraic sum of the forces ; the direction of the 

 resultant is the same as that of the greater of the parallel forces. 



The Centre of Gravity of a rigid body is the point upon which the 

 body could be supported or balanced; in other words, it is the point 

 through which the resultant of the parallel forces due to the weights 

 of the individual particles passes. 



Every material object has a centre of gravity, and the position of 

 this point for a particular object is the same so long as the object 

 retains the same form. 



The centre of gravity of such geometrical figures as circles, squares 

 and parallelograms is the centre of the figures, and can be determined 

 geometrically. The centre of gravity of unsymmetrical figures can 

 be determined by experiment. 



The centre of gravity of a triangle is located on the line drawn 

 from one of the angles to the middle point of the side opposite and 

 at a distance of one-third of this line's length from that side 

 of the triangle. 



Equilibrium. A body is said to be in equilibrium when all the 

 forces acting upon it balance one another. It is in stable equilibrium 

 when any turning motion to which it is subjected raises the centre 

 of gravity ; in unstable equilibrium when a similar motion lowers 

 the centre of gravity ; and in neutral equilibrium when the height 

 of the centre of gravity is unaffected by such movement. 



QUESTIONS ON CHAPTER VI. 



1. Describe an experiment to prove that the resultant of a number 

 of parallel forces is numerically equal to the sum of those which act 

 in one direction, less the sum of those which act in the opposite 

 direction. 



2. State the conditions for the equilibrium of three parallel forces. 

 Describe an experiment which shows these conditions. 



3. Apply the principle of moments to explain the conditions of 

 equilibrium for parallel forces. Give an example. 



4. What do you understand by the centre of gravity of a body ? 

 o. Describe an experimental method for finding the centre of 



gravity of any body, e.g., a waste-paper basket. 



6. Where is the centre of gravity of a triangle located. 



7. How could you find by a geometrical construction the centre of 

 gravity of a circular plate, a square piece of cardboard, or any other 

 geometrical figure ? 



8. When is a body said to be in equilibrium ? Distinguish between 





