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Pop Bottle Rocket, Part I: Action and Reaction

In this activity, students use pop bottle rockets to learn about Newton’s Third Law of Motion and how changing the action force results in a dramatic reaction.

2L pop bottle rockets are an excellent way to demonstrate the effects of forces on objects and provide many opportunities for observing, predicting, measuring and carrying out experiments while controlling for variables.

Part I of this series of activities demonstrates how a pop bottle rocket works.

You can do this activity on its own, or follow it up with Part II and/or Part III of this series for a deeper investigation into forces and rocket flight. 

How a pop bottle rocket works

A launcher powered by a hand bicycle pump or small compressor fills a plastic pop bottle rocket with compressed air. When the bottle is released from the launcher, air escapes the bottle. As the bottle pushes out the air, the air pushes the bottle upwards (Newton’s Third Law of Motion).

 Increasing the thrust (adding more pressure to the bottle) increases the acceleration. This is an illustration of Newton’s Second Law (a bigger force causes a bigger acceleration). We see the bigger acceleration as a higher flight.

Safety Tips:
Flying bottle rockets is a lot of fun, but be sure to take safety precautions seriously:

  • Rockets travel far; do not do this activity indoors.
  • Every launcher should be supervised by an adult.
  • Do not pressurize the rocket past 40 psi (pounds per square inch).
  • When pressurizing and launching the rocket, everyone should stand well away from the launcher. Make sure that all observers know that a rocket is about to be launched–a countdown and safety zone combination work well for this.
  • Watch the rocket’s entire flight to make sure it doesn’t hit anyone as it falls. 
  • Use only paper/cardboard fins, never metal. 
  • Wear eye protection. 

Objectives

  • Explore and demonstrate the effects of action and reaction forces.

  • Apply their understanding of forces and their effect on objects to manipulate the flight of toy rockets.

Materials

  • Per Group:
    a pop bottle rocket launcher – made (see instructions from NASA) or purchased (from an hobby /science supply like Boreal)

    a large field or open, outdoor space
    a 2L pop bottle
    materials to create fins and nose cones (e.g. scissors, tape, construction paper, foam, foil, cardstock)
    300–400mL of water
    safety goggles (for everyone)

Key Questions

  • Would larger or smaller soda bottles go higher?
  • Can we estimate how high the rockets go?
  • How could we measure and compare the heights of the different rockets?
  • How can we estimate or measure the speed of the rocket as it leaves the launcher?
  • How could we increase the pressure in the bottles before they took off?
  • How could we modify the soda bottles to get longer or higher flights?

What To Do

Preparation

  1. Make or purchase a pop bottle rocket launcher. There are several ways to build a launcher using standard materials from a hardware store. Whichever model you choose, be sure to use pressure rated PVC tubing. Instructions can be found in many online videos, or at: NASA | Water Rockets.
  2. Build a pop bottle rocket. Add fins and a nose cone to help it fly along a straighter path. Remember that the pop bottle is upside down when it’s launched, so design it with the opening at the bottom. Be careful not to cut into it— any holes or cracks in the bottle will make it impossible to pressurize.

Demonstration

  1. Review the safety measures before you begin.
  2. Set up the launch pad and pump firmly on the ground, with the launcher facing awayfrom the group and towards an open space. All observers should be at least 3–5m away from the launcher.
  3. Pour 300–400mL of water into the rocket.
  4. Lift the handle on the launch pad and carefully fit the pop bottle onto the nozzle. Ensure that the bottle is locked in.
  5. Find two volunteers to be the “launcher” and the “pumper.” Ask both to wear safety goggles.
  6. The “pumper” adds air using the pump. Be sure not to go past 40 psi (5–6 bike pumps) as greater pressure can rupture the bottle rocket.
  7. Start a countdown!
  8. At “blast off!” the “launcher” pulls back the handle, releasing the pressure and sending the rocket into the air.

Tips on using the launcher

  • A good seal is required to pressurize the bottle. If you see a spray of water as you pump, the seal is too loose.
  • Bottles become weakened from launching and landing. Although a new bottle can withstand more pressure, never pressurize a bottle above 40 psi. Retire water rockets after 10 flights or sooner if you suspect damage.

Extensions

  • Try different shaped bottles or putting different shaped fins on the bottle to make it fly straighter.
  • Divide the class into groups of 2 or 3 and allow each to design their own rocket, adding fins, a nose cone, and any other features they think will help the rocket fly further or straighter. Test the rockets to see what designs fly best.
  • Follow up with the activity Pop Bottle Rocket Part II: Projectile Motion. Experiment with the angles at which you launch the rocket. Which angle makes the rocket travel furthest?
  • Follow up with the activity Pop Bottle Rocket, Part III: Mass and Force. Experiment with the amount of water you add—what happens if you half fill the bottle with water? How far can you get a bottle to go?

Other Resources

Science World Resources | Pop Bottle Rocket, Part II: Projectile Motion

Science World Resources | Pop Bottle Rocket, Part III: Mass and Force