177 
17 IV. EFFECTS OF PRESSURE WAVES 
1. GENERAL PRINCIPLES 
IV. EFFECTS OF THE PRESSURE WAVE ON AN OBSTACLE 
1. MODE OF ESTIMATING EFFECTS 
In the pressure wave there are four physical magnitudes of interest: 
(a) pressure 
(b) forward particle velocity 
(c) momentum, of total magnitude [pudz (p= density, u = particle 
velocity, x = coordinate in the direction of propagation) 
(d) energy. 
The effect of the wave upon an obstacle, which we shall hereafter call the 
"target" can always be calculated in terms of the pressure exerted upon it by the 
water. To do this, however, we must know the extent to wnich the presence of the ob- 
stacle in turn modifies the pressure in the water. Because of this complication, it 
may be more convenient to consider the process in terms of one or more of the forego- 
ing magnitudes other than the pressure. The most advantageous choice of a mode of 
approach will depend largely upon the relation between the dimensions of the obstacle 
and the effective length of the pressure wave. 
Misconceptions may easily arise from carrying over into the dynamic field 
modes of thought that are appropriate to the static field only. The following gen- 
eral principles may be noted: 
A. Strength of materials may be of little importance in determining the ef- 
fects of explosions. For example, it is unimportant that a pressure of 10,900 pounds 
per square inch is required to rupture a metal structure if 50,900 pounds per square 
inch is available in the pressure wave. 
The action of explosives upon objects near at hand will depend more upon 
tneir relative inertia than upon their cohesive strength. At greater distances, on 
the other hand, cohesive strength may be the chief determining factor. 
B. The path of least resistance will not be favored by explosive forces to the 
same degree as by forces of smaller magnitude but longer duration. 
For example, a charge detonated in contact with a metal plate may punch a 
hole through the plate, although the path of least resistance would lie through the 
air. The air is accelerated outward with extreme rapidity by the high pressure, but 
the adjacent part of the plate is likewise given a considerable acceleration, suf- 
ficient to cause rupture. A dense object placed over the explosive, such as water or 
earth, increases the effect on the plate because of its inertia, the time of action 
of the explosive being thereby lengthened. Water on the opposite side of the plate, 
on the other hend, diminishes the effect somewhat. 
C. Large-scale effects tend to be very much less severe than the local effects 
close to the charge. This is a consequence of the short time of action of the forces. 
