# Homework Help: Quick question regarding bernoullis principle

1. Nov 4, 2007

### HelloMotto

Ok I've just started learning about Bernoulli's principle so I dont have that much knowledge of it..
so anyways I know that as the velocity of air or fluid increases, the pressure decreases. In order for the velocity of the fluid to increase, there has to be like a constriction thingy.. I've searched google and all the examples i got was a narrow tube.

so my question is where is the constriction on a wing of an airplane?

2. Nov 5, 2007

### dynamicsolo

The examples you found with the constriction are things like Venturi tubes. This particular application of B's Principle, BTW, is the central gimmick for devices like atomizers (such as spray can nozzles) and engine carburetors (which were in fact inspired by perfume atomizers).

Anything that forces an acceleration of the fluid will work this way. For an aircraft wing, the upper surface is convex, while the lower surface is (nearly) flat. Air flowing over the upper surface, as the wing is pulled through the atmosphere, is caused to speed up relative to the air flow on the underside, thereby lowering the air pressure on the upper surface. The result is a net upward force on the wing, called the "lift".

Such a cross-sectional design for a wing is referred to as an airfoil [try googling that for more info]. BTW, humans didn't invent that -- the cross-sections of birds' wings are shaped that way (da Vinci noted that over 500 years ago); it's just one more example of inspired human copying...

Last edited: Nov 5, 2007
3. Nov 5, 2007

### catkin

Teachers and text book writers have long misunderstood Bernoulli's law resulting in generations of confused students, some of whom have become teachers and text book writers and continued the misunderstanding.

For a comprehensive explanation of the misunderstanding see http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html [Broken] (thanks and acknowledgements to Weltner and Ingelman-Sundberg).

Misunderstanding arises because two key concepts are commonly overlooked. They are both expressions of Newton's Second Law of Motion $\textbf{F} = m\textbf{a}$, applied to a fluid:
• Acceleration of air is caused by pressure gradients. Air is accelerated in direction of the velocity if the pressure goes down. Thus the decrease of pressure is the cause of a higher velocity.
• Pressure gradients perpendicular to the streamlines are caused by the deflection of streaming air. The deflection of air generates regions of lower and higher pressure according to the curvature of the streamlines
Text books commonly derive Bernoulli's Law from “conservation of energy”. This is not wrong but, by ignoring the first key concept, allows Bernoulli's Law to be misinterpreted as saying that lower pressure is caused by a higher velocity. The next step in bamboozling the student is to give examples of the second key concept as examples of Bernoulli's Law in action, such as spin, levitating balls, lifting pages and how aeroplane wings work.

Last edited by a moderator: May 3, 2017