Have you ever seen someone blow smoke rings? Have you ever asked yourself why the smoke takes the shape of a donut as it passes through the air? In this experiment, a hole is cut in the bottom of a paint bucket and a shower curtain or trash bag is tightly laid over the other open end. When the shower curtain or trash bag is lightly tapped, there is a change in pressure inside the paint bucket. This change in air pressure causes air to rush out of the bucket in a ring or donut shape. We call this donut-shaped ring a torus.
The air on the inside is moving faster than the air on the outside of the ring, holding the donut shape as it travels through the air. The rush of air can be felt if the bucket is aimed at someone. In order to see the torus, we add fog to the inside of the bucket. Once the bucket is filled with fog, the donut-shapes can be observed shooting out of the bucket because the fog takes the torus shape. If the shower curtain or trash bag is tapped lightly, the torus can be observed and the air in the center can be seen traveling outward toward the outside of the ring. If you tap it harder, you can shoot smoke rings. Try to shoot targets with these rings!
Discussion of Grade Appropriate Content:
K-3 grade: At this grade, students know that solids, liquids, and gases have different properties and that the properties of substances can change when the substances are mixed, cooled, or heated. Students know an object’s motion can be described by recording the change in position of the object over time. They know the way to change how something is moving is by giving it a push or a pull. The size of the change is related to the strength, or the amount of force, of the push or pull. They also know that tools and machines are used to apply pushes and pulls (forces) to make things move. Tapping the plastic film at the bottom of the trash can causes air, which is a gas, to get pushed out of the small opening at the end of the trash can. Although we cannot see the movement of air directly because it is transparent, we can use smoke from a fog machine to see the air being pushed out of the trash can. Depending on how hard the plastic film is tapped, we can change how much force the air is given. If we use a strong force, the air leaving the trash can will be moving fast and the smoke ring will move quickly out of the end of the trash can. A lighter tap causes the air to move more slowly as a result of the force being small.
4 – 5th grade: Students at this grade level know the above content but they also know that air is made of molecules. The motion of these molecules can be observed when looking at the movement of the rings. They also know that water vapor in air moves from one place to another and can form fog or clouds, which are tiny droplets of water or ice, and can fall to Earth as rain, hail, sleet, or snow. The fog machine generates tiny droplets of water vapor. Those water droplets cause light to be deflected resulting in the vapor being cloudy.
Middle School: Students at this grade level know that energy can be carried from one place to another by heat flow or by waves, including water, light and sound waves, or by moving objects. The tapping of the plastic film generates a sound wave that moves the air out of the trash can. They also know that the average speed is the total distance traveled divided by the total time elapsed. In the case of a vortex ring, students can calculate the speed of the smoke ring by counting how many seconds (one-one thousand, two one-thousand,…) and dividing that time into the distance that the smoke ring has traveled. Demonstrators can have the students practice measuring the speed of a vortex ring by counting the number of seconds that the ring travels a distance across the field.
College-level content: The velocity of air leaving the trash can is fastest at the center of the opening at the bottom of the trash can and slowest at the edges of the circular opening. As a result, the air moves out away from the base of the trash can and also outward from the center of the hole. The net effect is the formation of a ring of moving air that can be visualized using smoke (actually, water vapor) generated by a fog machine.
Next Generation Science Standards
K-PS2-1. Forces and Interactions: Pushes and Pulls
Plan and conduct an investigation to compare the effects of different strengths or different directions of pushes and pulls on the motion of an object.
K-PS2-2. Forces and Interactions: Pushes and Pulls
Analyze data to determine if a design solution works as intended to change the speed or direction of an object with a push or a pull.
5-PS1-1. Structure and Properties of Matter
Develop a model to describe that matter is made of particles too small to be seen.