When friction converts kinetic energy into thermal energy, what effect does this have on the pressure of the liquid within the pipe?

• A. Increases
• B. Decreases
• C. Remains the same
• D. Oscillates

Answer: (B)

Friction in a flowing liquid causes energy to be lost as heat. That loss reduces the amount of mechanical energy available in the fluid, specifically the pressure energy that supports the static pressure. In a pipe with steady flow, the energy lost to friction shows up as a drop in pressure downstream, so the static pressure decreases along the length where friction acts. This is why long or rough pipes with significant flow rate exhibit a pressure drop.

It wouldn’t increase because friction is a dissipative process, and it wouldn’t remain exactly the same or oscillate in a steady flow—the energy loss manifests as a lower pressure downstream.