In Saving Private Ryan there are scenes where the soldiers are underwater and bullets are shown going into the water with bubbles trailing behind them.
In films with submarines, the propellers often have bubbles coming out.
Where are these bubbles, or the air inside them, coming from?
In: 5
The bubbles contain steam or water vapor. It is caused by *cavitation* which is when the local pressure in the water is so low that the water basically boils. With a propeller you are creating high and low pressure areas behind and in front of it. This pressure difference provides the thrust which *propels* the boat. If the low pressure falls below the vapor pressure for the water temperature, it boils and forms a bubble. This bubble will sustain until it moves into a higher pressure area where it will collapse. If it collapses near the propeller, the shock-wave can actually melt a small bit of metal and over time this can result in holes.
With bullets there is a low pressure area right behind it which causes similar bubbles.
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