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Momentum in fluids
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Fluid flow transfers momentum to surfaces causing forces.
Fluid jets change the momentum of the fluid hitting a surface, giving a force F = ρQv (rate of momentum change). This explains water jets, wind forces and jet engines.
A 2 kg/s water jet at 10 m/s stopping on a wall exerts 20 N.
Force depends on mass flow rate and speed.
In more detail
In fluids, momentum is carried by flowing material, and forces arise when a stream changes direction or speed. The momentum flux gives the force on pipe bends and nozzles.
Real-world uses
- Pipe bends are anchored to resist the momentum change of flowing water.
- Jet engines and nozzles use the momentum flux of fluid to compute thrust.
- A jet of water from a hose hitting a wall exerts a force because the water loses its momentum in the sideways direction.
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Frequently asked questions
Why do pipe bends need supports?
Because the flow changes direction and exerts a force on the bend from its momentum change.
Does pressure also matter?
Yes. Pressure forces add to the momentum flux force in many fluid problems.
Does a fluid exert force on a wall it flows against?
Yes. The momentum change of the fluid as it turns or stops creates a force on the surface.
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