FAA Aviation Mechanic General (AMG)Physics, Math and Fluid LinesMedium
Hydraulic fluid flows at 8 ft/sec through a line section with a cross-sectional area of 0.60 square inch. The line then necks down to a section with an area of 0.30 square inch. Assuming incompressible flow, what is the fluid velocity in the smaller section?
- A8 ft/sec
- B16 ft/sec
- C32 ft/sec
- D4 ft/sec
Show answer & explanationAnswer & explanation
Correct answer: B. 16 ft/sec
The continuity equation states A1V1 = A2V2 for incompressible flow. Solving: V2 = (0.60 × 8) / 0.30 = 4.8 / 0.30 = 16 ft/sec. As area decreases, velocity must increase to maintain the same flow rate.
Why the other options are wrong
- A. This assumes velocity stays unchanged, ignoring area change.
- C. This doubles the correct value using an incorrect ratio.
- D. This results from dividing rather than the correct proportional calculation.
Continuity Equation
For incompressible fluid flow, the product of cross-sectional area and velocity is constant along a line: A1V1 = A2V2.
- Flow rate (volume/time) stays constant
- Smaller area = higher velocity
- Larger area = lower velocity
Memory trick: 'Squeeze the hose, water shoots faster' — smaller area, higher speed.