58 ELEMENTARY GENERAL SCIENCE CHAP. 



The mass will therefore move with a velocity of 3^ feet per 

 second in the direction in which the 7 Ibs. was moving. 



EXAMPLE. Two masses of 6 and 10 Ibs., moving with velo- 

 cities of 4 and 5 feet per second respectively, collide with one 

 another. Find the final velocity, (a) if the bodies are moving 

 in the same direction ; (6) if they move in opposite directions. 

 The same equation is used as before, the only difference being 

 that in the first case the velocities are both taken as + , whereas 

 in the second case one is taken as - . We therefore proceed as 

 follows : 



Case ((.i) 



(6 + 10) V = (6x4) + (10x5) 



16 V = 24 + 50 = 74 



Hence V = 74-T-16 



= 10 feet per second. 

 Case (&) 



(6 + 10) V = (6x4) -(10x5) 



16 V = - 26 



Hence V = -26-J-16 



= - If feet per second. 



In this case the direction of motion is that of the mass of 

 5 Ibs. 



EXAMPLE. A bullet weighing 2 oz., and moving with a 

 velocity of 1,200 feet per second, is fired into a block of wood 

 weighing 2 cwt., and carries the wood onwards with it. With 

 what velocity does the wood move ? 



2 cwt. =3584 oz. 



Momentum after event = momentum before event 

 (2 + 3,584) V = (2x1, 200) + (3, 584x0) 



3,586 V = 2,400 + 

 .Therefore V = 2,400-r-3,586 



= 0'67 feet per second. 



Force. Suppose a body to possess a certain momentum, then 

 for the momentum to change or tend to change, something must 

 act upon the body, and that something is termed force. In 

 the words of Professor Hicks : When a gradual change of 

 momentum is either produced or tends to be produced in a 

 body, that body is acted on by force. 



It must be clearly understood that by thus defining force we 

 do not get to know anything more about it. Nobody can tell 



