Ceiling Fan and Balloon Science
Question: What causes a helium balloon to stay under a spinning ceiling fan? Click the right-pointing arrow below to watch the video.
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Answer: You probably know the basic part: It’s the air coming from the fan. The helium balloon wants to rise but the air blowing down from the fan keeps pushing down the balloon. Also, the swirling motion of air from the spinning fan blades pushes the balloon around in circles. But here’s the trickier part: Why doesn’t the balloon get pushed away? Why doesn’t the spinning air from the fan just blow the balloon across the room?
If you watch a balloon spinning like this for a while, you’ll probably assume that the balloon is trapped under the fan somehow. It is. This happens because faster-moving air creates less pressure than slower-moving air. Think of the air getting pulled apart—almost stretched—to move faster. As a result, the molecules in the air get more spread out. Because of that, there are fewer air molecules in any spot, and that creates less pressure.
So the air getting pushed down by the fan is moving faster than the rest of the air in the room, which creates a low-pressure area under the fan. That low-pressure area is what traps the balloon.
This is also part of the explanation behind flight. Airplane wings make air flow faster across the upper surface than the lower surface. So there’s lower-pressure air above the wing, and that let’s higher-pressure air under the wing push up an airplane.
TIPS:
- You’ll probably need the fan on high to keep the helium balloon pushed down
- Keep your hands away from the spinning fan blades
- This tends to work better if you keep some string on the balloon, almost like a tail on a kite. But be careful to keep that string from getting tangled in the fan’s blades.