Teacher Background
What It’s About
All vehicles, whether designed for land, sea, air or space, are governed by the scientific principles stated in Isaac Newton’s Laws of Motion. In brief, the laws are as follow.
First Law
An unbalanced force is required to cause an object to change its state of motion or rest. Once in motion, an object will continue moving in a straight line until acted upon by an unbalanced force. Imagine two people pushing on each other. If they are equally strong, neither will move because the opposing forces are balanced. If one person is stronger than the other, the forces are unbalanced and the weaker person will be pushed backward.
Second Law
An object’s acceleration is directly proportional to the force exerted on it and inversely proportional to its mass. In other words, the less mass an object has, the more that object will accelerate when it is acted upon by an unbalanced force. Acceleration also can be increased if the force is increased (f=ma).
Third Law
Every action is accompanied by an equal and opposite reaction. When force is applied to an object, the object exerts an equal opposing force. Consider what happens when someone fires a shotgun. The pellets fly out of the barrel and the shooter is pushed back by a strong “kick.”
Rockets are an excellent example of the Laws of Motion at work. This activity demonstrates all three laws. Students construct and test a lightweight “rocket” car propelled by the action/reaction force of air escaping from an inflated balloon. The escaping air exerts an unbalanced force on the car, shifting it from a state of rest to a state of motion. The force of the balloon squeezing on air inside accelerates the car when the air is released. Because the car’s mass is very low, it impedes the acceleration minimally. If the car were heavier, it would accelerate more slowly. Finally, the balloon’s wall exerts an action force on the air, causing it to shoot out the nozzle. This creates an equal and opposite reaction force that propels the car. When the balloon’s air runs out, there is no more force to push the car, which coasts until friction brings it to a stop.
Objectives and Standards
Students must answer the following question.
How can a balloon propel a race car?
Materials and Setup
Teacher Materials
Materials per Student
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2 coffee stirrers (axles)
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2 non-bending straws (sleeves for axles)
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2 round, 12.7 cm (5 in.) size balloons
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Flexible straw
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Masking tape
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Pair of scissors
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Sandpaper
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Sharp pencil
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Styrofoam™ tray (large, no sections)
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Copy of “Design Plan,” “Build a Rocket Racer,” and “Rocket Racer Data” pages
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Copy of the “Wheel Patterns” page on card stock
Optional: Obtain 26 plastic drink covers (4 per student) to serve as wheels.
Setup
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Construct a sample rocket car to demonstrate to students how to operate their vehicles.
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Create a “racetrack” with a 10-meter strip of masking tape. Use a marker to indicate one-meter intervals on the course.
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Obtain 24 Styrofoam™ trays (no sections).
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Make 24 copies of the “Wheel Patterns” page on card stock for students to use as templates when cutting wheels from the Styrofoam™ tray.
Procedure and Extensions
Time: 3 sessions to build, test and modify/retest
What to Do
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Announce to your class that this is the day of the big rocket car race. Each student will design and build a rocket car, and race his/her car on the track in the hall. Show your students a sample race car. Explain that this is only one possible design, and that their cars could look very different.
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Demonstrate how the car works. Inflate the balloon by blowing through the straw. Pinch the straw and set the car on the floor. Release the straw and away the car goes!
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Distribute the student sheets. Have students brainstorm design ideas as a group. Then have each student plan a car and draw pictures of what he/she wants his/her car to look like on the “Design Plan” sheet. Finally, have students begin construction on their cars.
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Give each student a large Styrofoam™ tray, explaining that the car and its wheels must be made from the tray. Show students how to cut the pieces for their designs. If you are not using scissors, demonstrate the pencil trick: outline the pieces by punching the Styrofoam™ with a sharp pencil tip. When the outlines are completely punched, break out the pieces. Smooth the rough edges by rubbing them against a hard surface. It is especially important to smooth the edges of the wheels. Have students use sandpaper to refine the edges. (Optional: Obtain plastic drink covers from a restaurant to use as wheels.)
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Show students how to create sleeves for the axles by cutting the non-bending straws. The sleeves should be shorter than the coffee stirrers.
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When students have drawn and cut out all their car pieces, show them how to create wheel sets. Press one end of a coffee stirrer (axle) through the center of a wheel. Extend the end of the stirrer about a centimeter through the other side, and hold it in place with a small piece of masking tape. Slip a non-bending piece of straw over the stirrer. Then attach the other wheel to the opposite end of the stirrer. The wheels should turn freely when you hold the axle by the straight straw covering the stirrer. Repeat this process for the second pair of wheels.
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When both wheel sets are complete, mount them on the bottom of the car platform. Use masking tape to hold them in place. Make sure the wheels are not pressed against the platform and that they turn freely.
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Pre-inflate the balloon once to make it easier to re-inflate when the car is finished. Insert the short end of a flexible straw into the balloon nozzle. Use masking tape to attach the balloon securely to the straw. Squeeze the tape around the straw to seal any leaks.
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Mount the straw and balloon to the upper surface of the car’s platform with masking tape. Be sure the long end of the straw extends off the back of the platform.
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Have students test their cars, and explain that they may not perform as well as expected. Allow time for students to make needed design improvements before the “official” race. Mention that wheels that are not round, don’t turn freely, or not mounted straight will affect speed and direction.
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When all rocket cars are ready, organize races on the track in the hallway. For each trial, have two entrants inflate their balloons and hold their cars just behind the starting line. After a short countdown, have students release their straws. The fastest, straightest-running car wins!
Wrapping Up
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Conduct a post-race talk show, during which racers explain their accomplishments. What worked? What didn’t? How did they solve problems? What’s the best rocket racer design?
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Ask students how they would redesign their rocket cars to improve performance.
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Lead a class discussion about the impact of friction on the performance of students’ rocket cars. Ask, What are some possible sources of friction? (Wheels not round or rubbing on the frame, rough “track” surfaces, balloon touching the floor or front wheels, etc.)
Extra
Have students design their ultimate rocket race cars.
Teacher Guide