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 →Applying the work-energy theorem to a fluid element moving through a pipe of changing cross-section and height gives Bernoulli's equation, P + ½ρv² + ρgh = constant.
Statement: For the steady, streamline flow of an ideal (incompressible, non-viscous) fluid, the sum of pressure energy, kinetic energy, and potential energy, per unit volume, remains constant along any streamline:
Proof: Consider an ideal fluid flowing steadily through a pipe of varying cross-section. Take a fluid element between cross-sections of area (at height , speed , pressure ) and (at height , speed , pressure ). In a time , a volume of fluid enters at 1 and an equal volume leaves at 2 (by conservation of mass, since the fluid is incompressible).
Work done by pressure forces: The fluid behind pushes the element forward at 1, doing work ; the fluid ahead pushes back at 2, doing work . Net work by pressure:
Change in kinetic and potential energy: The net effect (since the flow is steady) is as if a mass was moved from position 1 (height , speed ) to position 2 (height , speed ):
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