Action­Reaction! Rocket

Summary
Students construct a rocket from a balloon propelled along a guide string. They use this model to learn about Newton's three laws of motion, examining the effect of different forces on the motion of the rocket.

Engineering Connection
Engineers of all disciplines use their understanding Newton's laws of motion to quantify the "invisible" forces acting on all objects. Just like a ball can be twirled on the end of a string, satellites and spacecraft stay in orbit around the Earth due to the balance between gravitational and centripetal forces. Eventually, satellites slow down due to the miniscule drag in the upper atmosphere, to the point at which gravity pulls them out of orbit. To keep them in orbit, engineers exploit the second law by designing thrusters that burn fuel and expel it from the thruster. The spacecraft moves forward in an amount equal to the force of the gas leaving the thruster, causing enough movement to re­orient the path of the object and keep it in orbit.

Learning Objectives
After this activity, students should be able to:
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Understand practical applications of Newton's Laws of Motion

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Use the model of the balloon to understand the different forces that act on the rocket

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Collect data from the experiment and graph the results

Materials List
For each group:
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Plastic drinking straw

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Plastic bag, about the size of an inflated balloon

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Paper streamers

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25 ft. of fishing line (20­50g weight) or string (nylon [slippery] string works better than twine [rough])

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Long, tube­shaped balloon

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Tape measure or meter stick

Introduction/Motivation
Start with an in­class demonstration. For example, have a student or the teacher stand on a skate board and throw a basketball. What happens? Have a student or the teacher throw a basketball filled with lead weights or similar, very heavy object (this could be dangerous; be very careful not to fall). What happens? (Answer:
The student or teacher rolls backwards on the skateboard.)
Or, as another demonstration: Pass around three containers (such that students cannot see the contents), one filled with something light such as feathers, one filled with something heavy such as lead weights and one filled with something in the middle such as rice or grains. Ask students which one is heavier. Ask them what they think is inside. Tell them that the heaviest one is heavier because it has a higher mass.
Rockets and rocket­propelled flight has been in use for more than 2,000 years. People in ancient China used gunpowder to make fireworks and rockets. In the past 300 years, people have gained a scientific understanding of how rockets work. Now, aerospace engineers use their understanding to make rockets fly farther, faster, higher and more accurately. Our understanding of how rockets work arises from Sir Isaac
Newton's three laws of motion. It is important for engineers to understand Newton's laws because they not only describe how rockets work, they explain how everything that moves or stays still works!
This activity demonstrates all three of Newton's laws of motion. The focus of the activity is Newton's third law of motion, but the first and second laws are intrinsically involved with the motion of the rocket as well. The air pushing its way out of the balloon is an action force, and it causes an equal reaction, which is the movement of the balloon. The more air initially in the balloon, the further the balloon travels along the string because the action force is greater. By the same token, if there is only a small amount of air initially in the balloon, the balloon travels a shorter distance.
Simply stated, Newton's three laws of motion are:
Law #1: Objects at rest will stay at rest, and objects in motion will stay in motion in a straight line unless they are acted upon by an unbalanced force. (law of inertia)
Law #2: Force is equal to mass multiplied by acceleration. (
F
= m a )
Law