# Air velocity

#### david90

Say you have a 3 inchs diameter tube and u have a machine that sucks air from it with force A. Then you have 2 tubes that is binded together each with a 1.5 inch diameter and have the same machine sucks air from it with force A also. Would the air velocity of the 3 inchs tube be slower, faster or equal to the 2 1.5 inch diameter tube?

#### zoobyshoe

##### Bipedal Hairy Critter
Find the area of a 3 inch circle.
Then find the area of a 1.5 inch circle, double it and see if it the same as the area of a three inch circle.

In practise you would have to consider the differences n friction between the two systems, but I don't think that's what you're asking.

#### theriddler876

I think it would be slower, try the reverse, blowing through a straw for example, the air comes faster out of one of those coffe stirring things than it does out of a regular straw that is, lets say half the length of the stirring thingy

#### russ_watters

Mentor
Assuming you mean pressure, not force, velocity is dependent on force only. The diameter has nothing to do with it. So the velocities are the same - according to bernouli's equation.

#### theriddler876

I think the diameter would affect it since it focuses the energy into a smaller exit, sort of like a water gun, make the hole bigger, and the water wont have as much engergy, speed, and thus it won't go as far,

#### zoobyshoe

##### Bipedal Hairy Critter
If the force/pressure remains constant a change in diameter definitely has an effect on the velocity. The smaller the diameter of the tube, the faster the air must flow.

Two 1.5 inch diameter tubes do not have the same total diameter as one 3 inch diameter tube. This is an easy calculation. Do it and you will find out which is the larger, slower channel, and which is the smaller, faster one.

#### russ_watters

Mentor
Nope and nope. Think about it guys - given a specific pressure, what changes when the diameter changes? Theriddler: "it focuses the energy into a smaller exit" What energy? It may be a little tricky, but pressure is pressure. 1psi on a 1 square inch opening area is 1 pound of force. 1psi on a 1 square foot opening area is 144 pounds of force. 144x the force, but you are moving 144x the air, so it cancels out.

Bernouli's equation is P=1/2 rho*V^2
p=dynamic or velocity pressure
rho=density of air
V=velocity of air

This is a classic problem where the static pressure in a vessel is completely converted to velocity pressure in the opening. So to find velocity, enter pressure and density and solve for velocity. Area/radius is nowhere to be found.

#### zoobyshoe

##### Bipedal Hairy Critter
We can use Bernoulli's equation to find the pressure exerted by the moving fluid, not the pressure causing the fluid to move. We can use Bernoulli, for example, to find out how much less pressure the faster air on top of a moving aerofoil exerts than the slower air underneath.
But Bernoulli's says nothing about the pressure/force causing the fluid to move in the first place.

The relevant effect in a situation where you are considering a change in velocity due to a change in channel diameter is the Venturi effect. Venuri observed, in part,that constricting a flow of fluid caused it to speed up.

#### theriddler876

Originally posted by russ_watters
1psi on a 1 square foot opening area is 144 pounds of force. 144x the force, but you are moving 144x the air, so it cancels out.
but it's not converted at the end, the pressure to velocity, it happens at the very beginning

use a vacum for example, say it has on setting, when you turn it on it begins sucking, now if you attach one of those tubes to get the edges, the air moves in quicker and you can feel that there's more suction because the same amount of air has to go through a smaller space

here's a better example if you have a ballon and you fill it with air, then poke a needle the air that will go trough that hole created will move faster than if you take scissors and cut a big one on the top

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