- Forces and Motion
In this activity, students use pop bottle rockets to learn about the path of projectiles by testing rocket launches at different angles.
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 II of this series of activities demonstrates how different launch angles can affect the distance a rocket travels. A basic understanding of rocket trajectories will help you increase how far your rocket flies (its range). You can do this activity on its own or in combination with Part I and/or Part III of this series for a deeper investigation into forces and rocket flight.
If you’ve thrown a ball, you’re familiar with the curved path (trajectory) followed by a moving projectile. The shape of the trajectory of a projectile is probably familiar to you–it is a parabola (an upside-down U-shape).
When you launch a pop bottle rocket, it is moving both upward and forward. All other things equal (the mass and design of the rocket, the pressure at which it is released, and the weather conditions), the angle at which you launch your rocket will determine its range (the horizontal distance it travels).
For example, here are four different launch angles:
If you launch a rocket straight up, it goes very high but comes back to earth quite near the launching pad. If you launch it almost horizontally, gravity brings it down to the ground before it can travel very far. In between is an angle (45 degrees, in the absence of significant air resistance) that will let the rocket travel horizontally as far as possible before gravity brings it back to Earth.
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.
- Work in small groups to design, build, and test your own rocket. (Hint: Have a few launchers available so groups can test many angles.)
- 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?
- After doing both Part II and Part III, combine your discoveries to come up with an experiment that tests the best combination of launch angle and rocket mass.
What To Do
- 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
- 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.
- Choose 5 different launch angles between 0–90° to test. (90° = straight up, and 0° = horizontal). Make a prediction relating the launch angle to distance travelled end.
- Head to the launcher and pour 300–400mL of water into the rocket.
- Lift the handle on the launch pad and carefully fit the pop bottle onto the nozzle. Ensure that the bottle is locked in.
- Find four volunteers to be the “launcher”, the “pumper”, the “timer”, and the “measurer”. The launcher and pumper put on safety goggles, the timer receives a stopwatch, and the measurer, the metre sticks.
- The “pumper” adds 5 big pumps of air into the pop bottle. Be sure not to go past about 40 psi (5–6 bike pumps) as greater pressure can rupture the bottle rocket.
- The “launcher” moves the rocket to the desired angle using the protractor, counts down, and launches the rocket.
- The “timer” times the length of the flight from launch to landing and the “measurer” measures the range of the rocket and records it on the data sheet.
- Repeat for each of the angles, pumping the launcher to the same psi. Record all of the time and distance results.
- Discuss the results.