Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut each sandpaper sheet into eight equal pieces, one for each group.
2. Sand down the sharp tip of each wooden skewer, leaving most of the tapered portion. Alternately, each student group may blunt their own skewer.

Safety Precautions

Although latex (in balloons) is not considered hazardous, not all health aspects of this substance have been thoroughly investigated. Latex may be an allergen. If a balloon bursts, be careful of flying particles. The tip of the wooden skewer is very sharp. Blunt the tip with sandpaper. Wear safety glasses.

Disposal

Used balloons and straws should be thrown away in the regular trash. All other materials may be stored for future use.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Enough balloons are provided for each team to have six—one or two for testing the prototype, three or four for testing the new design, and one for the final challenge. The elasticity of the balloons decreases after each inflation, but a minimum of five trials should be able to be accomplished. Additional balloons are available in a package of 50 from Flinn Scientific, Catalog No. AP6420.
• Testing the alignment of the balloon car is best done on a tile floor if possible. Students can use the lines of the tiles to determine how straight the car travels. Otherwise, a series of meter sticks lined up end to end may serve as a guide.
• The chassis may be safely and easily cut with the plastic knives included in the kit. The sandpaper may be used to smooth or round the edges.

Teacher Tips

• This is a great hands-on activity for incorporating STEM, transfer of energy, Newton’s laws, thrust, engineering design and scientific inquiry.
• The specific design challenge may be changed from year to year or class to class. Other options may include the following: 
  — Race uphill at a specific angle 
  — Longest distance 
  — Carry the most weight for 1 meter
• Be sure to give students any design constraints you deem appropriate. May they increase the traction or size of the wheels? Larger wheels may be made by taping large plastic lids to the wheels, cutting a hole in the center of each. May they change the shape of the chassis? May they use a different straw? More than one balloon? How much time will be given to design the car? Will they be able to run a practice test once the design is final?

Answers to Prelab Questions

1. Consider a balloon-powered car that is designed to accelerate very fast.
   1. For each of the following variables—thrust, friction and mass—indicate whether the quantity should be high or low.

      Thrust high
      Friction low
      Mass low

   2. Explain the reasoning for your answers to Question 1a.

      Since thrust is the force that propels the car forward, one would want as much thrust as possible. Friction opposes motion, so friction should be reduced. More force is required to accelerate a more massive object, so an object with little mass would be easier to accelerate.

2. A balloon car traveled 4 meters in 6.7 seconds. What was the average speed of the car?

   Speed = distance/time   4 m/6.7 s = 0.6 m/s

3. What safety precautions must be taken during this activity?

   Do not overinflate the balloons. If a balloon bursts, be careful of flying particles. The tip of the wooden skewer is very sharp. Blunt the tip with scissors or sandpaper. Wear safety glasses.

Sample Data

Mass of car: 17 g

Answers to Questions

Part III. Design Challenge

The challenge is to design a balloon car that will cross the finish line of a 3-meter route first. Consider the forces involved as the car travels to determine with your team which variables you will change in designing your car.

1. With your team, consider the following:
   1. Write an equation to show the relationship between distance, time and speed of the car.

      Speed = distance/time

   2. What variables might affect the thrust produced?

      The size of the balloon, the elasticity of the balloon, the amount of air in the balloon, the type of straw (nozzle), etc.

   3. Identify sources of friction on the car.

      Contact between the wheels and the floor, between the wheels and axles, between the wheels and bushings, between the car and the air.

Post-Lab Questions

1. Calculate and record the speed of the car for each trial in the data table.

   See Sample Data table.

2. What is the average speed of the balloon car prototype?

   0.71 m/s

3. After you have tested the redesigned balloon car, answer the following:
   1. Describe the changes your team made to the prototype car and how the changes affected the car’s performance.

      Student answers will vary, but should include ways to reduce the mass of the car, reduce the friction and increase the thrust.

   2. Which variables that affected the balloon car’s performance were difficult to control?

      Answers may include the elasticity of the balloon, the exact amount of air in the balloon, etc.

4. Consider a balloon car that is traveling at a constant speed.
   1. Describe the forces acting on the car.

      The escaping air results in thrust, the force propelling the car forward. Gravity is pulling down on the car and the floor is pushing up. The force of friction is present in several places: the air against the car, the wheels against the floor, the wheels turning on the axles and between the sides of the wheels and the bushings.

   2. Taken all together, are the forces balanced or unbalanced?

      All the forces are balanced since the car is traveling at a constant speed, neither speeding up nor slowing down.

   3. Once all the air from the balloon is expelled, what will happen to the car?

      The car will slow down and stop.

   4. Explain your reasoning for the answer to 3c in terms of Newton’s laws.

      Newton’s first law states that an object in motion with a constant velocity tends to stay in motion maintaining that velocity unless acted upon by an unbalanced force. Newton’s second law indicates that in order for an object to accelerate (change speed or direction), a force must be applied. Since the force providing the thrust is no longer acting on the car, only the force of friction remains. This results in an unbalanced force opposing motion, which slows the car down.

Introduction

Have you ever inflated a balloon and let it go to watch it zoom all around the room? You can put that energy to work with a balloon-powered car. First build and test a prototype balloon car and then redesign the car to improve its performance. How fast or how far will the car go?

Concepts

• Newton’s laws
• Friction
• Engineering design

Background

Isaac Newton (1642–1729), expanding on ideas presented earlier by Galileo Galilei (1564–1642) and others, described three laws of motion. These laws explain how a balloon car works. Newton’s third law of motion states that for every action force there is an equal and opposite reaction force. When a balloon is inflated, the air molecules push against the inside wall of the balloon (action force) and the walls of the balloon push against the air molecules (reaction force, see Figure 1). The pressure inside the balloon is balanced by the tension of the balloon material and the atmospheric pressure on the outside of the balloon. When the balloon is released, the air is forced out the “nozzle —the open mouth of the balloon. The escaping air causes an imbalance of forces inside the balloon, since part of the wall that was pushing back on the air molecules is now missing (see Figure 2). This unbalanced force is known as thrust. The thrust propels the balloon in the opposite direction of the escaping air.

By attaching a balloon to a car with wheels, the thrust can be used to accelerate the car. How much the car accelerates may be explained by Newton’s second law. Newton’s second law of motion states that for a given force, the mass of an object is inversely proportional to its acceleration (any change in speed or direction), while for an object of specific mass, the force needed to accelerate the object is directly proportional to its acceleration (see Equation 1). In other words, if the same force were applied to two objects of different masses, the object with less mass would experience a greater acceleration than the more massive object. Newton’s first law of motion states that an object in motion with a constant velocity tends to stay in motion, maintaining that velocity unless acted upon by an unbalanced force. Friction is a force that opposes motion and is caused by contact between two surfaces. When one surface slides past another, sliding friction is experienced. When an object such as a wheel or a ball rolls against a surface, rolling friction results. Fluid friction, also known as drag, occurs when an object moves through a fluid such as water or air. No matter what type of friction is experienced, the force will oppose motion.

Experiment Overview

The purpose of this activity is to collect data and identify patterns in the motion of a balloon-powered car. The activity is divided into three parts. Part I describes how to assemble the balloon car prototype. In Part II the performance of the prototype car will be tested. After testing is complete, the design of the car will be modified to produce a balloon car that performs best for the design criteria and constraints given by the instructor in Part III.

Materials

Prelab Questions

1. Consider a balloon-powered car that is designed to accelerate very fast.
   1. For each of the following variables—thrust, friction and mass—indicate whether the quantity should be high or low.
   2. Explain the reasoning for your answers to Question 1a.
2. A balloon car traveled 4 meters in 6.7 seconds. What was the average speed of the car?
3. What safety precautions must be taken during this activity?

Safety Precautions

Although latex (in balloons) is not considered hazardous, not all health aspects of this substance have been thoroughly investigated. Latex may be an allergen. Do not overinflate the balloons. If a balloon bursts, be careful of flying particles. The tip of the wooden skewer is very sharp. Blunt the tip with sandpaper. Wear safety glasses. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Part I. Balloon Car Assembly

1. Rub the tip of the wooden skewer on a piece of sandpaper to blunt the end, leaving most of the tapered portion.
2. Place one plastic bushing onto the tapered end of the skewer as far as it will go.
3. Place one wheel onto the skewer and use the wheel to push the bushing all the way to the flat end of the skewer (see Figure 3).
   {12593_Procedure_Figure_3}
4. Place a second bushing onto the skewer and push it all the way to the wheel. The bushings should be close to, but not touching the wheel. Note: If the bushing is tight and difficult to push along the skewer, use a second wheel to help push it, then remove that wheel.
5. Place a third bushing, a second wheel, and a fourth bushing on the skewer. Push them along the skewer until the third bushing is 5 cm away from the second. Use the width of the foam chassis to adjust the placement of the wheels (see Figure 4). Note: The edge of a ruler may be used to push the fourth bushing along if needed.
   {12593_Procedure_Figure_4}
6. Spin each wheel to ensure it spins freely. Adjust the placement of the bushings as necessary to allow the wheels to spin with a minimum amount of wobble.
7. Once the wheels and bushings are placed the correct distance apart (see Figure 4), use scissors to score the skewer next to the fourth bushing. Once scored sufficiently, carefully break the skewer at the score. This makes one axle for the balloon car.
8. Use sandpaper to smooth the broken end of the axle and the broken end of the remaining skewer.
9. Repeat steps 2–7 for a second axle. Smooth the broken end of the axle with sandpaper.
10. Use a rubber band to attach each axle to the chassis (see Figures 5a and 5b).
    {12593_Procedure_Figure_5}
11. Attach a balloon to the short end of a flexible straw with a rubber band (see Figure 6). Wrap the rubber band around enough times to securely fasten the nozzle of the balloon to the straw so no air will leak out—but without crushing the straw.
    {12593_Procedure_Figure_6}
12. Thread the long end of the straw under the two rubber bands on the chassis so the open end is positioned beyond the end of the chassis and the flexible bend is at the other end.
13. Bend the flexible portion of the straw with the balloon upward (see Figure 7).
    {12593_Procedure_Figure_7}

Part II. Testing the Balloon Car

1. Mark a starting and finish line 3 meters apart on the floor.
2. Measure the mass of the finished balloon car and record the mass on the Balloon Car Challenge worksheet.
3. One group member should blow through the end of the straw and inflate the balloon. Caution: Do not overinflate the balloon. Everyone should be wearing safety glasses.
4. Place a thumb or finger over the end of the straw to prevent air from escaping.
5. Set the car on the floor with the front of the car at the starting line.
6. One person should be the timer who counts down to signal when the other partner should let go of the car.
7. Time how long it takes for the car to travel four meters.
8. Record the time and all observations about the motion of the car on the worksheet.
9. If the car does not roll in a fairly straight line, make any necessary adjustments.
10. Repeat steps 2–7 two more times. Note: The same person should inflate the balloon each time. Use a clean straw if another person inflates the balloon.

Part III. Design Challenge

The challenge is to design a balloon car that will cross the finish line of a 3-meter route first. Consider the forces involved as the car travels to determine with your team which variables you will change in designing your car.

1. With your team, consider the following questions:
   1. Write an equation to show the relationship between distance, time and speed of the car.
   2. What variables might affect the thrust produced?
   3. What variables might affect the force of friction on the car?
   4. What changes will be made in the car’s design that are within the design constraints given by the instructor?
2. With your team, plan, discuss, test and evaluate your design.
   1. List any safety concerns and the precautions that will be implemented to keep yourself, your teammates, and your instructor safe during testing.
   2. Consider the strengths and limitations of your design.
   3. How will the testing data be recorded?
   4. How will the data be analyzed to determine a successful design?
3. Throw used balloons and straws into the regular trash.

Student Worksheet PDF

12593_Student1.pdf