62 ELEMENTARY GENERAL SCIENCE CHAP. 



our faces. The difficulty of moving a heavy body, or giving it 

 momentum, is also a familiar example of the same property. 



The Attraction of Gravity at any place is an Example of 

 a Uniform Force. The general laws which have just been con- 

 sidered are well illustrated by the attraction of the earth upon a 

 mass at its surface. We have learnt to call this force of attrac- 

 tion by the name of the body's weight. Let a body containing 

 m units of mass be attracted to the earth with W units of force, 

 or, as we may just as correctly say, let the weight of in units of 

 mass be W units and call the acceleration which would be pro- 

 duced in it in the unit of time, if the mass moved towards the 

 earth, g units ; then by the equations given on p. 60 we can write 



W - mg 



W 



and m = 



which tells us that 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 

 one second in a body moving freely towards the earth. 



Value of "g." It has already been seen that a body moving 

 from rest with an acceleration of a units per second per second 

 travels over a distance s represented by the equation : s = .T at-. 

 Hence, in the case of a body moving towards the earth under the 

 influence of the force due to gravity, which causes a regular 

 acceleration of y units, the distance travelled over depends 

 upon the value of "</." But the distance moved over by 

 such a body moving from rest towards the earth in one second 

 can be measured. It has been found to be 16 feet. All that 

 has to be done, therefore, is to substitute this value in the 

 equation above, after changing the general symbol for accelera- 

 tion, a, into the particular acceleration, </, applicable to this case. 

 We thus have to substitute 16 for s and 1 for t in the equation 



s = -l- gfi which gives 



16 = ^ x <j x 1 from which 

 g = 32 or 



the acceleration due to gravity is equal to a velocity of 32 

 feet per second in every second. 



Motion of Bodies falling from Best. The consideration of 

 all questions concerning bodies falling from rest towards the 



