68 ELEMENTARY GENERAL SCIENCE CHAP. 



Inertia. The inability shown by a material body of itself to 

 change its condition of rest or of uniform motion is called its inertia. 

 Inertia may also be defined as the capacity of a body to possess 

 momentum. 



The Attraction of Gravity at any place is an example of a Uniform 

 Force. Let a body containing m units of mass be attracted to the 

 earth with W units of force and call the acceleration which would be 

 produced in it if the mass moved towards the earth rj units. Then, 



W=m f> (p. 62) 



ir 



and in 

 9 



The Weight of a body is equal to the number of units of mass it 

 contains multiplied by the number of units of acceleration produced 

 by gravity in a body moving freely towards the earth. 



The acceleration due to gravity is equal to a velocity of 32 feet 

 per second in every second, or of 981 cm. per second per second. 



Motion of Bodies falling from Best. Let r = velocity with which 

 such a body is falling, t the number of seconds through which it falls, 

 and g the acceleration due to gravity. Then, 



Parallelogram of Forces. If two forces acting at a point be re- 

 presented in magnitude and direction by the adjacent sides of a 

 parallelogram, the resultant of these two forces will be represented 

 in magnitude and direction by that diagonal of the parallelogram 

 which passes through this point. 



Resolution of Forces. A single force can be replaced by other 

 forces which will together produce the same effect. Such a sub- 

 stitution is called resolving the force, and the parts into which it is 

 resolved are called components. 



QUESTIONS ON CHAPTER V. 



1. Explain fully the circumstances which would justify us in 

 declaring the masses of two bodies to be equal. 



2. What is meant by momentum ? Define the unit of momentum. 



3. There are two bodies whose masses are in the ratio of 2 to 3 

 and their velocities in the ratio of 21 to 16. What is the ratio of their 

 momenta ? If their momenta are due to forces P and Q acting on 

 the bodies respectively for equal times, what is the ratio of 

 P to Q ? 



4. The masses of two bodies (P and Q) are in the ratio of 3 to 2 ; 

 the former is moving at the rate of 7^ miles an hour, the latter at 

 the rate of 200 yards a minute. Find the ratio of P's momentum to 

 Q's momentum. 





