Materials Included In Kit

Additional Materials Required

Safety Precautions

Use caution when launching the balloons. Be sure no one is in the path of the balloon rocket on the string before launching the balloon. The fishing line may be difficult to see. Be aware of your surroundings as you walk through the classroom. Inform students not to overinflate the balloons and cause them to pop. Flying balloon pieces may injure eyes. Wear safety glasses. Follow all normal laboratory safety guidelines.

Disposal

All the materials may be thrown into the normal trash. The straws, clothespins and fishing lines may also be saved and 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. This laboratory activity can reasonably be completed in one 50-minute class period if students prepare their procedure plan before coming to lab.
• It is highly recommended to tie the fishing line to two support stands at opposite ends of the classroom, if this is possible. Using support stands allows for easy removal of the fishing line and ensures no marks will be left on the walls. In addition, the support stands can easily be pulled apart and anchored to create a very taut fishing line. This keeps the balloons closer to the floor, making it easier for students to adjust them as needed. Make sure students pull the fishing line as taut as possible, without breaking it, for the best results.

• Inform students to blow up their balloons slowly to prevent them from popping. With each inflation, the rubber balloon will become weaker, thus limiting the number of times it can be inflated before it pops. Typically, around the fifth inflation the balloon may pop. Inflating them slowly will increase their lifetime.
• A weaker rubber balloon will produce less thrust. If a balloon has been inflated six or more times (and it has not popped), it may be time to use a new balloon.
• The “guidance” fishing line needs to be as taut as possible to prevent the balloon or straws from snagging the fishing line as it rockets across the room. Also, make sure the groups extend their fishing line parallel to the floor. Students may have an unfair advantage by using a downwardly inclined fishing line.
• Laboratory time may be saved by pre-cutting the fishing line to the proper classroom length before the lab. Cut enough string for each lab group.
• Remove all tall objects from the desks so that they will not interfere with the launch path of the rockets.
• Wall hooks may be useful for tying the fishing line if these are appropriate for your classroom walls.
• Holding the nozzle of the balloon directly against the launching wall and then releasing the balloon will help with the initial thrust of the balloon.

Teacher Tips

• Students should have some previous knowledge or experience with Newton’s third law of motion before performing this laboratory activity.
• For more advanced classes, have students develop a rocket that can make the round trip—one rocket engine shoots the rocket to one end of the classroom, in which the second rocket engine “opens” and sends the rocket back in the opposite direction. This is a very challenging proposition but is an excellent project for students who finish the experiment “early.” One suggestion is to tether a string to the second rocket engine to pull off a small clamp that seals the second balloon.
• Additional experiments could include the use of thinner straws, such as coffee stirrers or different size or shape balloons. Also, students could test whether their rockets can travel across the classroom at an uphill angle.
• Students may inquire why the Space Shuttle has exiting nozzles that fan outward into a bell-shape instead of inward into a small opening. The reason is that the exiting gases leaving the solid rocket booster of a Space Shuttle are traveling at supersonic speeds (faster than the speed of sound) as they reach the exit nozzle. When gas moves faster than the speed of sound it no longer follows Bernoulli's laws because it acts more like a solid. In a flared-out bell nozzle at the bottom of the rocket booster, the supersonic, non-Bernoulli-type gas accelerates even more.

Answers to Prelab Questions

1. Define Newton’s third law of motion in terms of how a rocket “works.”
   Newton’s third law states that for every action force there is an equal and opposite reaction force. The gas from the burning fuel pushes on the rocket chamber, and the rocket pushes on the gas particles. The gas ejected out of the bottom of the rocket causes the rocket to be thrust upward.
2. What are some variables or conditions that may affect the distance a balloon rocket travels? Suggest some possible modifications in the rocket or launch design that may improve the rocket performance. (Hint: The fishing line must be as taut as possible, without breaking, for the best performance.)
   If the balloon is not filled with enough air, then there will not be enough thrust to push it across the room. The balloon may get hung up on the fishing line if there is too much friction on the straw. The balloon may get hung up on the fishing line if it is not properly lined up to shoot straight across the classroom.
   
   To improve the performance of the balloon rockets, make sure the fishing line is taut so the balloon does not get hung up on the sag of the fishing line. Use as thin of fishing line as possible to limit the amount of friction produced. Be sure the straw lines up on the balloon so that the balloon will travel straight down the fishing line. The nose of the balloon should point toward the target wall and the nozzle opening of the balloon should point toward the fishing line attachment point on the launch wall.

Sample Data

Sample Procedure

1. Unravel enough fishing line to extend the entire length of the classroom. Leave approximately a meter of fishing line slack and then cut the fishing line with scissors.
2. Tape or tie one end of the fishing line to one wall. Make sure the fishing line is high enough to extend parallel to the floor across the classroom without interfering with objects in the room.
3. Obtain a drinking straw. Use scissors to cut it to approximately three inches.
4. Slide the drinking straw piece onto the string attached to the wall.
5. Extend the fishing line to the opposite wall. Loosely tape the end of the fishing line, or temporarily tie it, to the wall so that the fishing line is taut and parallel to the floor. Use a slip knot or clamp so the string can be undone and another balloon can be launched on the fishing line, if necessary.
6. Slide the straw piece toward the secured end of the fishing line.
7. Obtain a long, thin balloon. Carefully blow up the balloon to stretch it out, and then allow it to deflate.
8. Place two 5-cm pieces of masking tape on the straw piece. Place the midpoint of the tape on the straw so that the tape ends extend equally from each side of the straw (see Figure 5).
   {13256_Data_Figure_5}
9. Inflate the balloon to about ¾ full. Pinch the end of the balloon closed with fingers and twist it several times to seal the balloon. Use a clothespin to clamp the opening closed.
10. Tape the inflated balloon to the straw piece, making sure the balloon is balanced, the straw piece is lined up along the string and is not crooked, and the balloon is aimed down the string towards to opposite wall. Continue to pinch the opening closed with the clothespin so no air can escape.
11. Hold the balloon near the wall and line up the balloon to “shoot” straight down the fishing line.
12. When the balloon is in position, release the balloon opening. Do not give the balloon any extra push.
13. Launch results: How far did the balloon travel? Did it make it all the way to the opposite wall? If not, what modifications need to be made to achieve the goal of rocketing to the opposite wall? In a data table, record the results of the launch, the distance the rocket traveled, and any sources of problems and corrective action that is required.
14. If the rocket did not travel all the way across the room, make necessary modifications and repeat steps 5–13. It is best to remove the original balloon and tape from the straw piece, and then use new tape.
15. Repeat step 14 until the balloon rocket reaches its goal of traveling across the classroom on the fishing line.

Sample Data Table

Answers to Questions

1. Why does the balloon move when it is blown up and the pressure inside the balloon is released?
   The air is expelled out the opening and the air exerts an equal and opposite force on the balloon. The balloon pushes the air out, and the air pushes back on the balloon in the opposite direction.
2. Why is the air pushed out of the balloon?
   The stretched rubber exerts a force on the air inside the balloon as the stretched rubber relaxes back to its original size. The force from the relaxing balloon forces the air out the opening. The original force comes from the initial inflation process when air was forced into the balloon and the balloon stretched.
3. Write a paragraph describing how the balloon rocket performed. Did the balloon travel the entire length of the classroom? How many attempts were needed to achieve the desired goal? What were some major problems that needed to be solved before the balloon rocket traveled across the classroom? How were the problems solved? What aspects of the rocket or launch design were most helpful in achieving the goal of the rocket traveling the length of the classroom?
   Student answers will vary.
4. List some suggestions that might improve the performance of the balloon rocket.
   If students used straw pieces, they may suggest that one long straw may work better than two or more straw pieces. The use of thinner or smoother string may have reduced the frictional forces and prevented the balloon from slowing down. A larger, more streamlined balloon would have more thrust and may be more aerodynamic, thus limiting its drag. Inserting a rigid tube, such as a piece of a drinking straw, into the balloon nozzle may create a more uniform and constant thrust.

Introduction

Experiment with Newton’s third law of motion and launch a balloon rocket across the classroom.

Concepts

• Newton’s third law of motion
• Bernoulli’s principle
• Rocket engine thrust
• Friction

Background

Newton’s third law of motion states that for every action force there is an equal and opposite reaction force. Rockets clearly show Newton’s third law in action. When a rocket burns fuel, hot gases are forced out the bottom of the rocket at high speed. The fast-moving gas particles are pushed by the rocket chamber in one direction and the gas particles, in turn, push on the rocket in the opposite direction. A common misconception about rocket thrust is that when the fast-moving gas particles exit a rocket engine, the gas particles push against the air outside the rocket and this causes the rocket to shoot upward. However, if this were the case, then rockets would never work in outer space because there are no air molecules in space for the fast-moving gases to push against. Instead, the fast-moving particles are forced out the rocket engine by the body of the engine.

When the fuel burns, a great amount of heat is created and the pressure inside the rocket combustion chamber increases. At the same time, the walls of the combustion chamber push back on the fast-moving gas particles. Rockets are composed of strong, solid materials with a small opening at the bottom. This opening is the only region on the engine where the pressure can be released. Since gas particles move from high to low pressure, the gas shoots out the bottom of the rocket. This creates a net force that thrusts the rocket in the opposite direction of the ejected gases (see Figure 1).

An enormous amount of fast-moving gas particles need to be generated in order to lift a rocket into orbit. A small thrust channel increases the speed of the hot gases as they exit from the larger combustion chamber. Gases always accelerate toward lower pressure, so the high-pressure gas moves faster and faster as it rushes out of the nozzle. The constricted flow path increases the speed of the gas particles. This increase in particle speed in a chamber as the diameter decreases is an example of Bernoulli’s principle (see Figure 2). The small-diameter chamber increases the speed of the exiting particles and therefore increases the net force that blasts off the rocket. An enormous amount of fast-moving gas particles need to be generated in order to lift a rocket into orbit. A small thrust channel increases the speed of the hot gases as they exit from the larger combustion chamber. Gases always accelerate toward lower pressure, so the high-pressure gas moves faster and faster as it rushes out of the nozzle. The constricted flow path increases the speed of the gas particles. This increase in particle speed in a chamber as the diameter decreases is an example of Bernoulli’s principle (see Figure 3). The small-diameter chamber increases the speed of the exiting particles and therefore increases the net force that blasts off the rocket.

Experiment Overview

The purpose of this experiment is to investigate the variables involved in launching a “balloon rocket” across a classroom. Balloon thrust, friction and rocket “guidance” play crucial roles in the successful launch of a balloon rocket across the classroom.

Materials

Prelab Questions

1. Define Newton’s third law of motion in terms of how a rocket “works.”
2. What are some variables or conditions that may affect the distance a balloon rocket travels? Suggest some possible modifications in the rocket or launch design that may improve the rocket performance. (Hint: The fishing line must be as taut as possible, without breaking, for the best performance.)
3. Read the Experiment Overview and Materials sections. On a separate sheet of paper, write a step-by-step procedure for launching a rocket balloon on a string across the classroom. Include a data table to record information about each rocket launch. What distance did the rocket travel? What problems arose? How can the problems be addressed?

Safety Precautions

Use caution when launching the balloons. Be sure no one is in the path of the balloon rocket on the string before launching the balloon. The fishing line may be difficult to see. Be aware of your surroundings as you walk through the classroom. Do not overinflate the balloons and cause them to pop. Wear safety glasses. Follow all normal laboratory safety guidelines.

Procedure

1. Verify the procedure (see Prelab Question 3) with your instructor and review all Safety Precautions.
2. Carry out the procedure and record all data in a suitable data table.
3. Answer the Post-Lab Questions.

Post-Lab Questions

1. Why does the balloon move when it is blown up and the pressure inside the balloon is released?
2. Why is the air pushed out of the balloon?
3. Write a paragraph describing how the balloon rocket performed. Did the balloon travel the entire length of the classroom? How many attempts were needed to achieve the desired goal? What were some major problems that needed to be solved before the balloon rocket traveled across the classroom? How were the problems solved? What aspects of the rocket or launch design were most helpful in achieving the goal of the rocket traveling the length of the classroom?
4. List some suggestions that might improve the performance of the balloon rocket.