Windbag Wonders Experiment
- Unroll one of the Windbags and tie off one end with a simple overhand knot.
- Put the bag over your mouth, take your time, and blow three big breaths of air into it. Grab the bag in front of your mouth and squeeze your hand to close it off. Slide that hand forward down the bag pushing the air you blew into it toward the knotted end. How did you do? The bag is completely full. Right? Oh, not so much?
- Looking at the disappointingly inflated Windbag, estimate the number of breaths it would take to fill the bag. You can get a fairly decent measurement by holding up the “inflated” Windbag to an unrolled Windbag. How many breaths would it take for you? We’re going to show you how to do it in one.
- Have your assistant hold the knotted end of the Windbag at your mouth level. The bag should be horizontal and straight away from you. Spread out your fingers and hold the opening of the bag as wide open as you can get it.
- Keep your mouth about six inches from the wide-open end of the Windbag. Take a couple of deep breaths (don’t over do it or you risk fainting) and blow a single, comfortable breath of air into the bag (without touching it with your mouth).
- Quickly grab and seal the bag with your hand as you did before. Slide your hand forward until it stops. It’s likely you’ll be surprised! Now you can either tie off the end with a simple knot like you did before or push the air out from the other end and have your assistant give it a try. Compare this inflation with the first one and share a high five with your assistant. You both deserve it!
- An assistant
- Adult supervision
How Does It Work?
The bag quickly inflates because air from the atmosphere is drawn into the bag from the sides next to the stream of air from your lungs. For you science types out there, here’s the technical explanation. In 1738, a scientist named Daniel Bernoulli observed that a stream of fast moving air is surrounded by an area of low atmospheric pressure. In fact, the faster the stream of air moves, the lower the air pressure drops around it. When you blow into the bag, you create an area of low pressure inside the bag and higher pressure air around you in the atmosphere rushes into the bag to equalize things. In other words, air in the atmosphere is drawn into the bag at the same time you are blowing into it.
Take it Further!
- Build three-dimensional geometric shapes and giant balloon animals by joining several Windbags together. Use the rubber bands to arrange the tubes in various shapes. Make giant cubes, triangles, pyramids, or any other shape imaginable.
- Hook two rubber bands together by laying two on top of each other and pulling them through each other in opposite directions.
- Start by creating a Windbag triangle that lays flat on the floor. Use the rubber band “joints” to connect the three angles of the Windbag triangle.
- Connect the other 3 Windbags, one each, at the corners of the flat triangle.
- Once all of the corners are connected, bring the three non-triangle Windbags to a point above the middle of the triangle. Use the rubber bands to connect the three together and voila! You have a Windbag Pyramid.
Now you have a “joint” to connect two Windbags. Connecting the Windbags is as simple as wrapping the two rubber band loops around inflated Windbags. Once you have the hang of connecting inflated Windbags with rubber bands, let’s try making a pyramid. You’ll need 6 inflated Windbags.
- Rubber bands
- Adult supervision
Take it Even Further!
- Prepare two Windbags by tying a knot in one end of each bag.
- Spread the Windbags out flat on one table with the open ends hanging well over the opposite edges.
- Ask for several people to help you position another identical table upside down on top of the first table. The two Windbags should be sandwiched in between the tops of the two tables.
- Ask each helper to kneel down by the ends of the table and begin blowing into the bags. Remind them to squeeze the Windbags closed after each breath and not to let any air escape.
- The force of the air in the Windbags will slowly cause the inverted table to rise!
- Two desks or tables
- Two assistants
- Adult supervision
How Does It Work?
The science behind the lifting table can be explained by Pascal’s Law. His experiments with fluids led him to a conclusion that the pressure exerted on a confined fluid (in this case the air in the bag) exerts equal pressure in all directions. In other words, the compressed air is exerting pressure underneath the inverted table equally throughout the long Windbag. This same principle is being applied when you pump up a bicycle tire or when an auto mechanic uses an air lift in a garage.
Firefighters use Bernoulli’s principle to quickly and efficiently force smoke out of a building. Instead of placing the fans up against the doorway or window, a small space is left between the opening and the fan in order to force a greater amount of air into the building. Firefighters call this “Positive Air Flow.”
Science Fair Connection
Demonstration Bernoulli’s principle is pretty cool, but it isn’t a science fair project. You can create a science fair project by identifying a variable, or something that changes, in this experiment. Let’s take a look at some of the variable options that might work:
- Determine how much Bernoulli’s principle is affected by the size of the Windbag’s hole. Devise a contraption the adjusts the hole size accurately or a clever way of measuring the hole size.
- Test how well the Windbags are able to hold onto the air inside of them. Is it affected by temperature? What about color?
That’s just a couple of ideas, but you aren’t limited to those! Try coming up with different ideas of variables and give them a try. Remember, you can only change one thing at a time. If you are testing different liquids, make sure that the other factors are remaining the same!