Apart from inertial force, which of the following forces is most important in motion of submarines under water?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2 MARCH 2017 Morning Shift
Detailed Explanation
Hydrodynamic forces on submerged vehicles depend on whether the vehicle operates on the free surface or deeply beneath it. Free-surface wave generation is governed by Froude number, whereas submerged resistance is governed by Reynolds number.
Why “Viscous force” Is Correct
When a submarine is deeply submerged, there are no surface gravity waves created at the air-water interface. The primary resistance opposing motion is skin friction and wake turbulence, making viscous force the dominant governing force.
Why the Other Options Are Wrong
- B. Gravity force: Gravity forces govern surface waves and are dominant for surface vessels (Froude model law), not deeply submerged submarines.
- C. Compressive force: Compressive forces are negligible because water is practically incompressible at submarine cruising depths.
- D. Surface tension force: Surface tension is negligible for large-scale engineering vessels (Weber number >> 1).
Key Point
Deeply submerged bodies (submarines, torpedoes) are governed by viscous forces and Reynolds number similarity.
Key Concepts for Students
- Reynolds number Re = ρVL / μ represents the ratio of inertial force to viscous force.
- Surface ships require Froude scaling; deeply submerged submarines require Reynolds scaling.
- Form drag and skin friction drag both originate from fluid viscosity.
Practice more: this is question 50 in Properties of Fluid MCQs – Part 4. Try the whole set and check every answer instantly.