Object with constant speed does not lose kinetic energy when faced with resistance

[scilearner]

Hello,

 

"Potential Energy (PE) + kinetic energy (KE) = Total Energy

 

When air travels through an airway at a CONSTANT FLOW RATE, the total energy of the fluid (PE + KE) decreases because friction converts some of this energy into heat. Since the fluid is moving at a constant flow rate, the loss is in potential energy (decrease in hydrostatic pressure)."

 

 

I don't understand this. Shouldn't the moving object lose K.E. Shouldn't resistance decrease speed.

 

I think the reason for that is I don't understand what is hydrostatic pressure, I researched but I don't get it.

 

"As we move down the airway, the tubes begin to narrow. When a tube narrows, fluid velocity increases causing an increase in KE at the expense of PE (ie. a compensatory decrease in hydrostatic pressure). This is referred to as the BERNOULLI EFFECT."

 

How does the fluid velocity increase when the resistance also increases when the tube narrows. Is the pressure inside this tube, exerted by moving molecules (KE) or hydrostatic pressure. I don't get it.

Edited November 8, 2009 by scilearner

[swansont]

Look at the capitalized phrase: constant flow rate. You have the same number of particles (so the same mass) moving at the same speed — the kinetic energy is constant from the parameters of the example.

 

pressure:

http://hyperphysics.phy-astr.gsu.edu/HBASE/press.html

[J.C.MacSwell]

Hello,

 

"Potential Energy (PE) + kinetic energy (KE) = Total Energy

 

When air travels through an airway at a CONSTANT FLOW RATE, the total energy of the fluid (PE + KE) decreases because friction converts some of this energy into heat. Since the fluid is moving at a constant flow rate, the loss is in potential energy (decrease in hydrostatic pressure)."

 

 

I don't understand this. Shouldn't the moving object lose K.E. Shouldn't resistance decrease speed.

 

I think the reason for that is I don't understand what is hydrostatic pressure, I researched but I don't get it.

 

"As we move down the airway, the tubes begin to narrow. When a tube narrows, fluid velocity increases causing an increase in KE at the expense of PE (ie. a compensatory decrease in hydrostatic pressure). This is referred to as the BERNOULLI EFFECT."

 

How does the fluid velocity increase when the resistance also increases when the tube narrows. Is the pressure inside this tube, exerted by moving molecules (KE) or hydrostatic pressure. I don't get it.

 

Those I bolded are very different concepts. Bernoulli's equations assume no friction, they assume an inviscid fluid, so they have to be modified in this case.

 

The velocity increases as the tube narrows due to the pressure gradient.

[scilearner]

Thank you both for help. So what is the pont of the bernoulli effect when it is not true. I mean when you constrict the pipe the molecules travel slower in real.

[J.C.MacSwell]

Thank you both for help. So what is the pont of the bernoulli effect when it is not true. I mean when you constrict the pipe the molecules travel slower in real.

 

Constrictions generally add drag to the flow, but at the constriction itself the molecules must speed up for the flow to stay at the same rate. If the cross sectional area gets smaller, and the flow rate is the same, the velocity must have increased.

 

The Bernoulli effect is certainly true - as the velocity speeds up the pressure drops. It does this in a very specific way according to Bernoulli's equation, unless there are any significant shear effects, friction or turbulence, which must be accounted for.