Build A Vortex Cannon

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Materials

  • Disposable plastic or paper cups (one eight-ounce and one 16-ounce size)
  • Scissors
  • Adult helper
  • Balloons (2)
  • Toilet paper
  • Duct tape
  • Measuring tape
  • Pen
  • Paper
    Materials needed for the Build a Vortex Canon activity

Prep Work

  1. Tape one strip of toilet paper to a door frame.
  2. Use the measuring tape to make a mark on the floor about 1 foot (ca. 0.5 meter), 3 feet (ca. 1 meter), and 6 feet (ca. 2 meters) away from the door frame with the toilet paper.
  3. Draw a four-centimeter- (1.5-inch-) diameter circle on the bottom of both of your cups.
  4. With the help of an adult, use the scissors to cut out both of the circles.
    A person is cutting out a circle from the bottom of a paper cut using scissors.
  5. Inflate two balloons, then deflate them again. This makes them easier to stretch later.
  6. Tie a knot in the neck of the balloons, then cut off the tip of the balloon.
    A person is cutting off the tip of a deflated balloon using scissors.
  7. Cover the top of the cup with the balloon with the tied neck facing outward.
    A balloon is stretched over the opening of a cup with its tied neck facing outward.
  8. Make sure to tape the balloon tightly to the cup on the sides—you want an airtight seal!

Instructions

  1. Choose the smaller cup to begin with. Practice firing the air cannon by pulling back on the knot of the balloon and releasing it. Try pulling the balloon the same distance every time to create the same amount of force that pushes the air out.

    Think about:
    What happens if you do this? Can you hear a sound? Do you feel anything in front of the cup?
    Person holding the built vortex cannon in one hand and pulling back on the knot of the balloon with the other hand.
  2. Stand at the half-meter mark in front of the toilet paper and shoot your air cannon aiming at the hanging toilet paper.

    Think about:
    Does the toilet paper move? Do all of the pieces move or only some of them?
    Person aiming the small vortex cannon at the strip of toilet paper.
  3. Next move to the one-meter mark and shoot your air cannon at the toilet paper.

    Think about:
    Does the toilet paper still move? More or less than at the half-meter mark?
  4. Finally go to the two-meter mark and again aim your air burst at the toilet paper.

    Think about:
    Do you still see the toilet paper moving? Why or why not?
  5. Change to the bigger vortex cannon and repeat the steps.

    Think about:
    Do your results change depending on the size of your air cannon? If yes, how? Can you shoot farther with the smaller or bigger cup? Why do you think this is the case?
    Person aiming the large vortex cannon at the strip of toilet paper.

Cleanup

Dispose of the cup and the balloon in your regular trash.

What Happened?

Could you make the toilet paper move by shooting an air vortex at it? The air vortex, which is basically a blast of air, shoots out of the cup so fast that it pushes the toilet paper away, so it starts moving. The air-vortex cannon works by quickly applying force to the air molecules inside the cup. When the balloon surface snaps forward it collides directly with the air molecules inside the cup, pushing and accelerating them toward the hole at the end of the cup. This push starts a chain reaction of high-speed collisions with other air molecules, and the only way for them to escape is through the opening at the end of the cup. You should have heard the “poof” of the air escaping rapidly through the hole each time you use your air cannon.

The fast-moving air then mixes with the still air outside the cup, and a vortex is formed. Because there is a higher air pressure on the outside of the ring (as the surrounding air moves slowly) and lower pressure on the inside (as the air in the vortex moves faster) the vortex spins and stays together as it travels across the room. Eventually the doughnut-shaped ring breaks up and disappears. You should have noticed that with a larger vortex cannon it takes longer for the air vortex to disappear, meaning that it traveled farther. This is because you are able to push a larger volume of air out of the cannon. Other design criteria such as the size and shape of the exit hole or the shape of the container (cup versus box) matter, too. Also, if you put more force on the membrane by pulling the balloon farther out, you make the vortex travel farther as the air gets pushed out of the cup faster.

Digging Deeper

Although air is invisible, it is made up of different molecules, mostly those of oxygen, nitrogen and carbon dioxide. When wind is blowing in your face you can actually feel these molecules press against you. But what makes them move to create wind? Air moves due to differences in air pressure. It always moves from areas of high to low air pressure. Differences in air pressure can, for example, be caused by temperature differences. You can also physically move air and create areas of high and low air pressure by “pushing” or manipulating the air around you. The investigation of how and why air flows is called the science of aerodynamics. One fascinating airflow phenomenon is a toroidal vortex, which looks like a doughnut-shaped ring of air. You rarely notice these unless there are some particles in the air such as steam or smoke, which make the vortex ring visible.

In this activity you created such an air vortex yourself with your homemade vortex cannon! With a vortex cannon you create a burst of air that shoots out of a hole. The fast-moving air displaces the air outside of the hole, which then swirls around in the shape of a doughnut. The air forms this shape because the air leaving the cup at the center of the hole is traveling faster than that leaving around the edge of the hole. The difference in air pressure between the fast-moving air inside the vortex ring and the slow-flowing air on the outside of it makes the vortex spin, keeping it stable while moving through the air. With some practice you can get your air vortex to move so fast that you might even be able to knock over a paper cup with it!

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