Q.State and prove Bernoulli's theorem. OR State and prove Newton's Law of Cooling.
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Start your 14-day free trial to unlock the full solution →Bernoulli's theorem is the work-energy theorem applied to a flowing fluid: pressure energy + kinetic energy + potential energy per unit volume stays constant along a streamline.
Statement: For the streamline flow of an ideal fluid (non-viscous, incompressible) that is steady, the sum of pressure energy, kinetic energy, and potential energy per unit volume remains constant at every point along the streamline:
where = pressure, = fluid density, = flow speed, = height above a reference level.
Proof (using the work-energy theorem):
Consider an ideal fluid flowing through a pipe of varying cross-section. Take a fluid element between cross-sections (at height , pressure , speed ) and (at height , pressure , speed ). In a small time , a volume of fluid enters at section 1 and an equal volume leaves at section 2 (by continuity/incompressibility, ).
Work done by pressure forces:
- Work done ON the fluid by pressure at section 1 (pushing it in):
- Work done BY the fluid against pressure at section 2 (pushing out):
- Net work done on the fluid element by pressure forces:
Change in kinetic energy:
Change in potential energy:
…
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