Answer: the stress is 384 pounds per square inch.
Step-by-step explanation:
Let S represent the stress in the material of the pipe.
Let P represent internal pressure of the pipe.
Let D represent internal diameter of the pipe.
Let T represent the thickness of the pipe.
The stress in the material of a pipe subject to internal pressure varies jointly with the internal pressure and the internal diameter of the pipe and inversely with the thickness of the pipe. Introducing a constant of proportionality, k, the expression becomes
S = kPD/T
The stress is 100 pounds per square inch when the diameter is 5 inches, the thickness is 0.75 inch, and the internal pressure is 25 pounds per square inch. It means that
100 = (k × 25 × 5)/0.75
125k = 100 × 0.75 = 75
k = 75/125 = 0.6
The equation representing the relationship becomes
S = 0.6PD/T
If the internal pressure is 40 pounds per square inch, the diameter is 8 inches and the thickness is 0.50 inch, then the stress would be
S = (0.6 × 40 × 8)/0.5
S = 384