Do you think you could build a car powered by nothing but air? A balloon-powered car is pushed forward by air escaping from a balloon, and it is fun and easy to build with materials you already have around your house. Can you imagine how you would want your own balloon-powered car to look? Can you design a car that will travel as far as possible? You can even measure your car's speed using your smartphone and a special sensor app. Get ready to grab some simple supplies to bring your idea to life!
Balloon-powered cars, like the ones in Figure 1, are fun to build and even more fun to play with. In this project you will be challenged to build and test your own balloon-powered car. A balloon-powered car consists of three main parts:
- The body of the car (piece of cardboard or plastic bottle in Figure 1)
- The wheels of the car (CDs or plastic bottle caps in Figure 1)
- The axles, which connect the wheels to the body, and allow the wheels to spin (wooden skewers in Figure 1)
Continue reading to learn more about the physics of how a balloon-powered car works.
Figure 1. Some examples of balloon-powered cars made from different materials.
Have you ever blown up a balloon and then let it go, without tying it shut? The air rapidly escapes from the balloon, making it zip all over the room! This is because when you blow up a balloon, you increase the air pressure inside the balloon. This air pressure stretches out the rubber balloon material, just like stretching a rubber band. Both the air pressure and the stretched rubber store potential energy, or energy that is "waiting" to do something. See Figure 2.
Figure 2.An inflated balloon stores potential energy in the forms of air pressure and stretched rubber.
When you let the balloon go, the rubber contracts, and air is rapidly squeezed out the opening of the balloon. The potential energy inside the balloon is converted to kinetic energy, or energy of motion, of the fast-moving air through the opening. Because the air is pushed out rapidly backwards, there is a reaction force that pushes the balloon forward, as shown in Figure 3. This principle comes from Newton's third law of motion, which states "for every action, there is an equal and opposite reaction."
Figure 3. According to Newton's third law of motion, when air is pushed backwards out of the balloon (out the end with t the opening), there must be an equal and opposite reaction force that pushes the balloon forward (the end opposite the opening).
When you just let a balloon go on its own, it tends to randomly fly around the room, and is almost impossible to steer. However, when you attach the balloon to a car, like the ones shown in Figure 1, you can harness the balloon's energy to propel the car forward! The engineering goal in this project will be to design, build, and test a car that is powered by nothing but a balloon. You want to design your car so it can travel as far as possible. For the ultimate test, you can follow a strict set of design rules used in the 2015 Engineering Challenge (see the Procedure for details).
- Body (of a car)
- Wheel
- Axle
- Air pressure
- Potential energy
- Kinetic energy
- Reaction force
- Newton's third law of motion
- Design requirements
- Iteration
- What is potential energy?
- What are some different ways potential energy can be stored?
- What is kinetic energy?
- What is Newton's third law of motion?
- What are some common materials and designs used to build balloon cars?
If you want to challenge yourself and compare your car's performance to other students' cars, you can follow the guidelines in the 2015 Engineering Challenge where you can only use the materials listed here. The maximum allowable quantity for each item is listed in parenthesis.
- CDs (4)
- Latex balloons, 9 inch (2)
- Jumbo straws, approximately 1/2 inch diameter and 9 inches long (2).
- Note that these are also referred to as "milkshake straws" or "smoothie straws." They are available from Amazon.com.
- Wooden pencil, 7 3/8 inch long (2)
- Sheets of paper, 8.5 x 11 inch (10)
- Jumbo paper clips, 1 3/4 inch long (6)
- Scotch® tape (1 roll)
- Scissors (allowed as a tool only, not a construction material)
If you just want to build a car for fun, or for a science fair project, you can pick which materials you want to use. Many different materials will work, but here are some suggestions:
- Something to form the "body" of the car, such as a plastic bottle, plastic cup, piece of Styrofoam®, cardboard box, or piece of corrugated cardboard
- Round objects for wheels, like CDs, bottle caps, or empty rolls of tape
- Axles, such as wooden pencils or skewers
- Material to connect the wheels to the axles, like a dry sponge or modeling clay
- Plastic drinking straws, used to connect the axles to the car's body, and to attach the balloon to the car
- Construction materials to hold things together, such as glue, tape, binder clips, paper clips, and rubber bands
- Balloons
- Cutting tools like scissors and a hobby knife
- Open area to test your car
- Lab notebook
- Tape measure
- Optional: Digital camera or smartphone to document your design process
- Optional: Smartphone with a sensor app such as phyphox, available for free on Google Play for Android devices (version 4.0 or newer) or from the App Store for iOS devices (iOS 9.0 or newer).