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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


Figure I. Figure II. Figure III.


Equipment: Wooden box with dimensions of 26 inches by 26 inches by 15 inches. This box has been custom made with a 10 inch diameter circular hole cut into its side. The back of the box has been removed and replaced with a large neoprene sheet. (See Figure I.)

Smoke generator

Step 1: The smoke generator is used to fill the box with smoke. The demonstrator strikes the back of the box (on the neoprene sheet) with his hand in a quick motion. A large smoke ring goes shooting out the front of the box and travels across the room! (See Figure II. and Figure III.)

Note that cigarette smoke or frozen water vapor from liquid nitrogen can be used if a smoke generator is not available. For health reasons, Phun Physics does not use cigarette smoke during a performance.


Basic Ideas: A stationary object will not move unless it is acted on by an unbalanced force.

Step 1: When the smoke has filled the box, the smoke is more or less motionless. There is little force acting on the particles of smoke. When the demonstrator strikes the back of the box, the neoprene sheet pushes inward. The particles of air and smoke inside the box experience a force from the neoprene sheet, which causes many of the particles to rush out of the box. This rush of air particles causes the smoke ring to travel across the room.

The physics behind aerodynamics is very complex, and to some extent, not fully understood. The basic idea behind the formation of the smoke ring is that the air rushing out the center of the hole is moving faster than the air exiting the hole near the edge. As the air tries to exit the hole near the edge, it is slowed down. This creates a circular motion around the edges of the hole which generates the smoke ring.

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Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia