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Waves and Optics
have students hold their palm in front of their face.
Purse lips and blow. What do you feel?
Now don't purse lips and "huff". What do you feel?
How? Air is always from a 98.6-ish body. Gas is expanding/cooling adiabatically with pursed lips. Air is compressed as it passes through your lips then expands into the atmosphere.
But how does the conservation of energy fit in here? Have we gotten something for nothing? The universe says, "NO!"
The proof is with this further activity:
Again purse lips. This time use your super-sensitive tongue to feel temp differences inside your mouth.
Blow hard while running your tongue back and forth from your lips to the rear of your mouth. You should feel the air warming when it is compressed as it enters your lips from the inside of your mouth.
This can be related to why fountain drinks come out cold, why clouds form, Prandtl-Glauert condensation (REALLY COOL!), etc.
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