Level: Grades 4 – 8. Time Required: one to three 45 minute sessions.
- To investigate Newton’s third law of motion by designing and constructing rocket-powered racing cars.
- To experiment with ways of increasing the distance the rocket car travels.
Students construct racing cars from Styrofoam food trays and power them with the thrust of an inflated balloon. In three racing trials, the racers shoot along a straight course, and the distance the racers travel is measured. Between trials, students redesign their racers to improve their performance and solve any “mechanical” problems that crop up. At the conclusion of the activity, students submit a detailed report on their racer design and how it performed in the trials.
The rocket car is an excellent demonstration of Newton’s Third Law of Motion. Air is compressed inside a balloon that is expanded. When the nozzle is released, the balloon returns to its original un-inflated size by propelling the air out of its nozzle. The action force of the expelling air produces a reaction force that pushes the racer in the opposite direction. The racer’s wheels reduce friction with the floor, and the racer takes off down the race course.
Although the rocket racer seems simple, there are many challenging complexities in its operation, and having students grapple with these can help them better appreciate the design work of engineers. The engineering design of the racer is very important. In principle (Newton’s second law of motion), the less mass the car has, the greater its acceleration will be. Generally, heavy rocket racers do less well than lighter racers. However, very small racers are limited by other factors. Vehicles with short wheel bases tend to circle or partially lift off the floor. Balance becomes a problem. The mass of the balloon may cause the car to tilt nose down to the floor, causing a poor start. Many designs are possible, including wide, narrow, and I-beam shaped bodies and three, four, or even six wheels.
Because of individual variations in the student cars, they will travel different distances and often in unplanned directions. Through modifications, the students can correct for undesirable results and improve their cars’ efficiency. They will have to review the trade-offs of their design. For example, an extra-long body may provide a straighter path, but the car might travel a shorter distance as a result.
NATIONAL SCIENCE CONTENT STANDARDS
Unifying Concepts and Processes
- Change, constancy, and measurement
Science as Inquiry
- Abilities necessary to do scientific inquiry
- Position and motion of objects • Motions and forces
Science and Technology
- Abilities of technological design
National Mathematics Content Standards
- Number and Operations
- Data Analysis and Probability
National Mathematics Process Standards
- Problem Solving
- Reasoning and Proof
MATERIALS AND TOOLS
- Styrofoam meat tray (request donations from local supermarkets)
- Small plastic stirrer straws (round cross section) – 2 per racer
- Flexi-straws – 3 per racer
- 4- or 5-inch round balloon
- Masking tape
- Sharp pencil
- Scissors (optional)
- Meter stick or metric measuring tape for laying out race course
- Student Work Sheets (one set per pair):
- 10 Meter tape measure or other measuring markers for track (one for the whole class)
Guidelines for the teacher:
This activity can be done individually or with students working in pairs or small groups. Allow 40 to 45 minutes to complete Part I of the activity. The activity stresses technology education and provides students with the opportunity to modify their car designs to enhance performance.
Refer to the materials list and provide what is needed for making one rocket car for each pair of students. Styrofoam food trays are available from butchers in supermarkets. They are usually sold for a few cents each, or you may be able to get them donated. Ask for thicker trays (about 3/16” thick). Yellow trays used for poultry work well. Waffle-bottom trays are also acceptable. Students can also save trays at home and bring them to class.
Double check the taping of the balloon to the straw. The balloon should be completely sealed, or it will be difficult to inflate, and some of its thrust will be lost through the leaks. Pre-inflating the balloon will loosen it and make it easier to inflate through the straw.
Lay out a race course in a large open space or hallway. The space can be carpeted, but textured carpets interfere with the movements of the racers. The cars will work very well on tile floors and carpeted floors with a short nap. Several tables stretched end to end will also work, though the cars may roll off the edges.
Stretch out a 10 meter-long line of masking tape and mark 10-centimeter intervals. If you have a 10 meter tape measure, just tape it to the floor.
Guide students through the redesign process to improve their racers. If their racers are not running well, ask them what they think the problem is. Then, ask them what they can do about it. Typical problems include having wheels too tight to the sides of the cars (friction), wheels or axles mounted crooked (racer curves off course), and axles not mounted in center of wheel or wheels not round (like “clown car” wheels).
The optional Part II of the activity directs students to design, construct, and test a new rocket car based on the results of the first car. If using this second part, provide each group with an extra set of materials, and save scraps from the first styrofoam tray to build the second car.
Although this activity provides one car design, students can try any car shape and any number, size, and placement of wheels they wish. Long cars often work differently than short cars.
1. Explain the activity to the students. Provide them with the How to Build a Rocket Racer Sheet. Go over the construction steps and demonstrate how to snap or cut out parts, mount the wheels, and attach the straw to the balloon.
2. Review the Rocket Racer Data Sheet and make sure students know how to fill out the graphs and what data they should collect.
3. Students should plan the arrangement of parts on the tray before cutting them. If you wish to avoid using scissors with younger students, they can trace the pattern pieces with the sharp point of a pencil or a pen. The pieces will snap out of the styrofoam if the lines are pressed deeply.
4. Lay out a track on the floor approximately 10 meters long. Several metric tape measures joined together can be placed on the floor for determining how far the cars travel. The students should measure in 10 centimeter intervals.
5. When student racers are ready, have one or two students at a time inflate their balloons and pinch off the end of the straw to keep the air inside. Have them place their racers just behind the starting line and release the straws. Regardless of how much curving a racer does, the measured distance is how far along the straight line of the race course the car reached.
6. Post distance records to motivate students to modify their racers to set new records. Provide guidance as students work on their improvements.
7. After each racer runs three times, have students complete their data sheets and sketch their final design on the design sheets.
If a second car is constructed, distribute the Rocket Racer Design Sheet so students can design their cars before starting construction. Students should design, construct, and test the new rocket car based on the results of the first car. Provide each group with a new set of materials, using the saved scraps from the first styrofoam tray to build the second car.
Depending on the time allotted for this activity, and the level of students, teachers may wish to direct the students to experiment with different car shapes and any number, size, and placement of wheels they wish. Long cars often work differently than short cars. To provide inspiration, have students view the variety of cars designed and built for the 1999 NASA Balloon Car Contest.
Students should submit for your review their Rocket Racer Data Sheet, describing the test runs and modifications that improved their car’s efficiency. Use this report for assessment, along with the Rocket Car Design Sheet and new car, should you undertake the second part of this activity.
- Have students write an explanation of Newton’s third law of motion using their rocket racers as examples.
- Hold Rocket Racer drag races on a 3-meter-long course. The fastest car is the one that crosses the finish line first. Calculate racer average speed by timing start to finish with a stopwatch (e.g., four seconds to go three meters = 0.75 m/second or 2.7 km/h).
- Have students try multiple balloons for additional thrust. How will students design cars that are balanced with the extra load?
- Have students control the thrust of their balloons by inflating them to the same diameter each time. How can students ensure that the balloon is always the same?
- Using the same materials, what other devices can be created that demonstrate the action- reaction principle of Newton’s third law of motion?
- Explore additional rocket activities and lessons through NASA’s Rockets Educator Guide, from which this activity is drawn.