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Old Jul 30th 2013, 02:23 AM   #1
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Bernoulli's Principle

Hi all,

In my textbook, when applying Bernoulli's Principle to a water pipe, the work done by the pressure on one end would be positive and negative on the other end. But how can one say the direction of the force exerted on an imaginary surface is either along or opposite to the direction of water flow if the pressure at a point has no specific direction?
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Old Jul 30th 2013, 06:18 AM   #2
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Originally Posted by Justusphung View Post
Hi all,

In my textbook, when applying Bernoulli's Principle to a water pipe, the work done by the pressure on one end would be positive and negative on the other end. But how can one say the direction of the force exerted on an imaginary surface is either along or opposite to the direction of water flow if the pressure at a point has no specific direction?
Without seeing the specifics of what yout text book is trying to explain, remember that work is calculated as the dot product of the force vector and the displacement vector. If displacement is measured in the direction opposite to the direction of force the resulting calculation for work is negative.

Last edited by ChipB; Jul 30th 2013 at 10:53 AM.
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