10 Outrageous Cartoon Moments That Use Real Physics

Wall-E's Fire Extinguisher Propulsion System
Wall-E uses the thrust of the extinguisher to propel himself in the opposite direction of the nozzle spray. Screen capture by HowStuffWorks staff

When Wall-E needs to leave a self-destructing escape pod in a hurry, he uses a fire extinguisher as a propulsion system, rocketing his way to safety. Wall-E was relying in Newton's third law of motion, which is commonly rendered as, "Every action has an equal and opposite reaction." More accurately, all forces result from the interaction between two objects, and when two objects interact, they apply an equal amount of force to each other, with the forces acting in opposite directions. A bat applies force to a baseball, and the baseball applies an equal amount of force to the bat, but in opposite directions. The difference you see in movement is due to Newton's second law of motion (a=F/m, commonly rendered as F=ma), which shows that objects with a lot of mass don't accelerate as much. When you bounce a tennis ball off a brick wall, the wall does accelerate, but if you plug its mass into the second law equation, the acceleration is so small you don't notice it.

In Wall-E's case, the two objects were Wall-E himself (including the fire extinguisher, which he was holding onto tightly) and the compressed gas inside the extinguisher. When he activates the extinguisher, the gas shoots out with a certain amount of force. An equal amount of force pushes Wall-E in the opposite direction.

Is this plausible? While fire extinguishers vary greatly in the amount of gas they contain and how much pressure they're under, it's definitely within reason that a large extinguisher could propel Wall-E at impressive speeds, especially considering Wall-E's low mass.

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