Marshmallow Puff Tube
Experiment with cardboard tubes of different lengths to see how far you can blow a marshmallow.
- One file folder (or other stiff paper or lightweight cardboard)
- Scissors
- Masking tape or transparent tape
- One or more full-sized marshmallows
- A few spoonfuls of flour
- Cut a rectangle from the file folder, measuring about 11.5 x 7.5 inches (29.5 x 19 centimeters).
- Place one of the long edges of the file folder inside the other and tighten to form a tube that fits around the circumference of a marshmallow. The tube should be snug around the marshmallow, but not so tight that the marshmallow isn't able to move. It may be easier to make the tube if you first pull the folder over the edge of a table to give the material a slight curve.
- When the tube has been rolled to the appropriate size, tape it once so it stays rolled, then tape the entire length of the seam.
Roll the marshmallow in flour, then shake it or tap it to remove any excess (this will help prevent the marshmallow from sticking to the tube).
Place the marshmallow in one end of the tube. Hold the other end of the tube up to your mouth, parallel to the floor, and blow hard into the tube. Notice how far the marshmallow goes.
Again place the marshmallow in one end of the tube, but this time put your mouth up to the same end of the tube where the marshmallow is located. Blow hard against the marshmallow itself, so that it has to travel the length of the tube before exiting. Be sure to keep the tube horizontal, and keep blowing the whole time the marshmallow is in the tube. Did the marshmallow go farther this time?
If you blow and the marshmallow won't move, check the diameter of the tube. It may either be too tight (in which case friction prevents it from moving) or too loose (in which case air blows right by the marshmallow instead of pushing it).
While the marshmallow is in the tube, your blowing increases the air pressure in the tube, creating a force on the marshmallow. As long as this force is greater than the friction force, there's an unbalanced force on the marshmallow. According to Newton's second law, F = ma, an unbalanced force accelerates an object. The speed of the marshmallow will keep increasing for as long as the marshmallow experiences an unbalanced force.
As soon as the marshmallow leaves the tube, your blowing no longer affects it. But the faster the marshmallow is traveling when it leaves the end of the tube, the farther it will travel before hitting the ground.
When the marshmallow is at the far end of the tube away from your mouth, it falls out of the tube almost as soon as you blow on it. The unbalanced force on it doesn't last very long, so the marshmallow doesn't accelerate very fast or travel very far.
When the marshmallow is at the end of the tube that's closest to your mouth, it experiences an unbalanced force for the entire length of the tube as you blow on it. Since the force acts for a longer time, the marshmallow is going faster when it leaves the tube, and it therefore travels farther.
The tube length that will provide maximum speed is determined by how long you can keep blowing strongly enough to maintain enough pressure in the tube so that the force produced on the marshmallow is larger than the friction force. If you have really powerful lungs, you can use a very long tube—and get a very fast-moving marshmallow!
Try using a significantly longer tube—double or triple the length of the file-folder tube. One-inch, Schedule 40 PVC pipe works well for this and eliminates problems you might encounter trying to tape file folders together.
Try elevating the tube at different angles above the horizontal to see the effect it has on the marshmallow's range.
What's the absolute maximum range you can achieve for the marshmallow? What combination of tube length and elevation gives this range? Do the results vary from person to person?
Here's a different challenge: The photo below was taken by Dean Baird during a demonstration by Don Rathjen and Paul Doherty at the fall meeting of the Northern California-Nevada American Association of Physics Teachers at Gunn High School, Palo Alto, California, on November 6, 2005.
In Dean's caption for the photo on the NCNAAPT Web site, he noted that the camera exposure was 1/30th of a second, and he posed the challenge of estimating the speed and range of the marshmallow, taking into consideration the non-zero launch height. Taking simple measurements from this photo and the exposure time provided, can you determine the speed of the marshmallow at the time the photo was taken? (Helpful hint: The blow gun is made from a file folder as described in this activity.)
