Manchester Memoirs, Vol. xlvi. (iQor), No. 3. 9 



and IF the velocity with which the pressure-wave travels 

 in the unstressed bar is 



/+«_/+^ Ip.a _E I r^ ^ n 

 t - a ' V -^ p' \l E:a V : 



This, as is well known, is the velocity of sound in an 

 elastic bar, E being the dynamic modulus of elasticity, 

 and }x the mass per unit of length and cross section. 



V- a \/ --^Sl P.a P 



We are now in possession of the principle which governs 

 the phenomena of blows. 



Suppose that a bar of steel of one-inch sectional area 

 were dropped on a rigid anvil with a velocity V oi 10 feet 

 per second, then, as 



77/ / ^0,000,000 . '?2'2 ft. . 1 2 ins. . , 



**^=K -, = 204,000 mch= 17,000 



V 0-277 pounds 



feet per second, we have 



V 10 



P=E . -Ttt = ^0,000,000 . = i7,6';opounas =-- 7-9 tons. 



IV ■^ ' 17,000 I' ^ t- 



It will be noticed that the pressure of the blow is 

 quite independent of the length of the bar. If a rod 

 moving with a velocity V comes in contact with a solid, 

 its front end is of course brought to rest at once, but its 

 tail end is still moving forward with a velocity V until 

 the pressure-wave has reached this point, then the whole 

 bar is at rest ; but an instant later the tail end is moving off 

 again with a velocity V due to the pressure P, and a 

 wave of negative velocity is now travelling towards the 

 anvil. As soon as this is reached the whole bar is 

 travelling away from the anvil with a velocity V. This 

 is a simple case of an elastic blow. 



We have already found the pressure of a blow. The 

 duration of a blow is equal to the time which the pressure- 



