# Why does fluid flow faster in a narrow tube?

1. Jul 14, 2013

### threy

Fluid flows faster in a narrow tube which results in low pressure and high pressure in a large tube?

2. Jul 14, 2013

### Staff: Mentor

If you have a fluid going from a large pipe to a narrow pipe (or vice versa), the fluid has to flow quicker in the narrow pipe to get the same flow rate ([strike]volume[/strike] mass per time).

Last edited: Jul 15, 2013
3. Jul 14, 2013

### gauss44

This is correct. And I'll add that there is a distinction to be made between this (a single tube with variations in width), and multiple (different) tubes.

In the later scenario: If you have 2 separate tubes which are not connected, there is no guarantee that water will flow faster through a narrower tube.

4. Jul 14, 2013

More correctly, the mass flow rate must be maintained (continuity). Volumetric flow rate only works here for an incompressible fluid.

5. Jul 19, 2013

### tiny-tim

hi threy!
pressure is not only force per area, it is also energy per volume

(1 Pa = 1 J/m3)

therefore conservation of energy requires that if the kinetic energy increases (ie if the speed increases), then the pressure must decrease

(mathematically, this is Bernoulli's equation … P + 1/2ρv2 + ρgh = constant along any streamline)

6. Jul 19, 2013

### Redbelly98

Staff Emeritus
Or put another way: the fluid speeds up when it enters a narrower portion of the tube. Since it speeds up, it has an acceleration, therefore a net force, in the direction it is moving. This net force must result from a higher pressure behind the fluid (in the larger tube portion) and a smaller pressure ahead of the fluid (in the smaller portion).

7. Sep 20, 2014

### john bastian

but why the fluid go faster in narrow tube than a thicker tube?

8. Sep 20, 2014

### CWatters

Perhaps it's this simple...

If the fluid is incompressible the flow rate (in cubic meters per second) must be the same at all points along the pipe. What goes in must come out.

If the cross sectional area (in square meters) changes the velocity (in meters per second) must change to maintain the same flow rate.

9. Sep 20, 2014

### enorbet

10. Sep 20, 2014

Right.