Answer :
We are given that the stress on a pipe varies directly with the internal pressure and internal diameter and inversely with the thickness. This means that the relationship between the variables is as follows:
[tex]S=k\frac{PD}{e}[/tex]Where:
[tex]\begin{gathered} S=\text{ stress} \\ P=\text{ internal pressure} \\ D=\text{ internal diameter} \\ e=\text{ thickness} \\ k=\text{ constant of proportionality} \end{gathered}[/tex]Now, we determine the value of "k" using the data provided:
[tex]\begin{gathered} S=100psi \\ D=5in \\ e=0.75in \\ P=25psi \end{gathered}[/tex]Now, we plug in the data:
[tex]100psi=k(\frac{(25psi)(5in)}{0.75in})[/tex]Now, we solve for "k". First, we solve the operations:
[tex]100=k(166.6)[/tex]Now, we divide both sides by 166.6:
[tex]\begin{gathered} \frac{100}{166.6}=k \\ \\ 0.6=k \end{gathered}[/tex]Therefore, the function is:
[tex]S=(0.6)\frac{PD}{e}[/tex]Now, we determine the value of "S" for the following data:
[tex]\begin{gathered} P=65psi \\ D=9in \\ e=0.8in \end{gathered}[/tex]Now, we plug in the data:
[tex]S=(0.6)(\frac{(65psi)(9in)}{0.8in})[/tex]Solving the operations:
[tex]S=438.75psi[/tex]Therefore, the stress is 438.750 pounds per square inch.