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Physics Behind a Hot Air Balloon


Have you ever wondered how a hot air balloon lifts in the air? Buoyancy, a basic principle, is used to generate lift for the hot air balloon. There are many parts to the hot air balloon in order for this to take place.
Image result for parts of a hot air balloon
different parts of a hot air balloon
The burner, usually consisting of power of several megawatts, is set underneath the opening for the envelope. It generates heat which is used to lift the balloon by way of buoyant force. Since the hot air produced inside the balloon is less dense than the air outside, it causes the balloon to lift off the ground. The principle that is responsible for this to work is known as Archimedes' principle. It states "that any object (regardless of its shape) that is suspended in a fluid, is acted upon by an upward buoyant force equal to the weight of the fluid displaced by the object." The same applies to air as it does water. 


picture of object floating in a fluid illustrating Archimedes principle


This figure shows an object submerged in water. This is the same result of a hot air balloon with the air instead of water. The center of buoyancy is on point C. 

V of object = displaced V of the fluid 

FB (upward buoyant force) = weight of the displaced V of the fluid V 


In a hot air balloon, FB is equal to the cooler air around the ballon. In order for the balloon to be lifted, the FB must be more than the weight of the heated air + passengers+ anything on the ballon and also the balloon itself. Otherwise the ballon with not have enough force to lift off the ground. 

Since the weight of the balloon is always at the basket, this is directly under the points C and G in the figure above, resulting in the balloon remaining upright and not rotating around.

To lower the ballon, the operator simply stops the burner from producing any more hot air, or by opening a small vent at the top of the envelope which releases the hot air already inside. 



Equation involved: 

P = ρRT 

P is the absolute pressure of the gas, in Pa 
(~ same pressure as the outside air) 

ρ is the density of the gas, in kg/m3 

R is the gas constant, in Joules/kg.K 

T is the absolute temperature of the gas, in Kelvins (K) 





Works Cited
"Hot Air Balloon Physics." Real World Physics Problems. N.p., n.d. Web.
"How Does a Hot Air Balloon Fly?" Wonderopolis. N.p., n.d. Web.

Limited, Fubra. "HOW THE BALLOON WORKS." How a Hot Air Balloon Works. N.p., n.d. Web.








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