a:  piston ~
b:  a giant tennis racket
What:  
"Microscopically speaking, compressing a gas must involve a surface, let's say a piston, charging into the gas. During this process, the piston is like a giant tennis racket smashing back all the trillions of tennis balls, i.e. the gas molecules, that happen to collide into it."
Useful?
Writer:  mrchuakh
LCC: 
Where:
Date:   Apr 15 2017  8:05 PM
# 11135 Critique Analogy
