Why does a blow through a tube get weaker even when there is no other way for the air to get out?

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Why does a blow through a tube get weaker even when there is no other way for the air to get out?

In: Physics

Fluids under pressure have a certain amount of potential energy. Friction between the fluid and the walls of the tube take some of that energy away. So the fluid has less pressure at the end of the tube than it did at the start, because of friction losses.

The losses are worse in a smaller tube. In bigger tubes, less of the fluid is in contact with the walls.

The tube has resistance to flow. The smaller the tube, the more the resistance. That’s why fire hydrants have really big connections. The resistance comes from friction of the water between water and the walls of the pipe, and between the water molecules.

Interestingly, it may also have to do with the velocity of the flow. Blowing harder will increase the velocity, which in turn will make the flow more turbulent and less laminar, which in turn will increase the friction and slow the flow at the other end.

Pressure is defined as force over area.

If you blow through a tube that is smaller than the opening in your lips would be, the air will get *stronger,* because the same force is being distributed over a smaller area.

But if you blow through a larger tube, the same force is getting distributed over a larger area and therefore feels weaker.