28 FORCE AND MOTION. 



many pounds suspended by the cord or rope would give it an equal 

 pull or tension. 



(&.) As we shall see, the force of gravity exerted upon a given 

 mass is variable. A given piece of iron would weigh more at the 

 poles than at the equator. Other variations in the force of gravity 

 are known. When, therefore, scientific accuracy is required, it will 

 not suffice to speak of a force of ten pounds, but we may speak of a 

 force of ten pounds at the sea-level at New York City. The neces- 

 sary corrections may then be made. But for ordinary purposes, 

 these details may be disregarded. 



68. The Absolute Unit. TIxe absolute or ki- 

 netic unit of force is the force that, acting for 

 unit of time upon unit of mass, will produce 

 unit of velocity. 



If we adopt the common units, the kinetic unit of force 

 is the force that, applied to one pound of matter for one 

 second, will produce a velocity of one foot per second. 



(a.) In all kinetic questions the kinetic unit is far more convenient. 

 Gravity units may easily be changed to kinetic units. At the lati- 

 tude of New York, the force of gravity acting upon one pound of 

 matter left free to fall will give it a velocity of 32.13 ft. per second 

 for every second that it acts. Consequently, at such latitudes, the 

 gravity unit is equal to 32.16 kinetic units. 



69. The Dyne. Instead of using a unit of force 

 based upon the foot and pound, scientific men are coming 

 to use a similar unit based upon the centimeter and gram. 

 This unit has recently received a definite name. The 

 dyne is the force which, acting for one second 

 upon a mass of one gram, produces a velocity of 

 one centimeter per second* 



(a.} If a body weighing 25 grams acquires in one second a velocity 

 of 30 cm. the moving force was 750 dynes. If it acquires the same 

 velocity in 2 seconds, of course tlie force was only half as great, or 

 375 dynes. 



(&.) The several units based upon the centimeter, gram and second, 



