How Everything Works
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QUESTIONS AND ANSWERS
 
Question 1188

Could you suspend a car on hot air produced below it? — RM, Toronto, Ontario
For the buoyancy of hot air to suspend a car, you would need a lot of it—in effect you would have to turn the car into a hot air balloon. That's because the lifting force experienced by hot air is really supplied by the cooler air around it and this upward buoyant force is proportional to the volume of hot air being lifted. Since a car is pretty heavy, the volume of hot air required will be enormous.

However, if you trap the air underneath the car, so that its volume can't increase, and then heat that air, its pressure will rise. This increased pressure below the car would produce an overall upward pressure force on the car and could support the car's weight. In effect, you would be creating a ground-effect hovercraft in which the elevated pressure of trapped hot air supports the weight of the vehicle. But it would be easier and less energy-intensive to pump air underneath your hovercraft with a big fan. That's what most ground-effect vehicles do. They pack extra air molecules underneath themselves and then allow those molecules to support their weight. Furthermore, because air molecules are always leaking out from beneath the vehicle, you'll need a fan to replace them anyway.

         

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