4. Alka Rockets

Purpose

To design a paper rocket propelled by alka seltzer and water to demonstrate Newton's third law of motion.

Background

The paper rocket in this activity is propelled according to the principle stated in Isaac Newton's third law of motion: "For every action there is an opposite and equal reaction." Gas pressure builds inside the film canister due to the mixing of alka seltzer and water. This action continues until enough pressure builds to blow apart the canister from its lid. The reaction is the launch of the rocket.

Materials

Index card (5 x 8 in.); empty film canister with lid that snaps inside; markers, crayons, or colored pencils; tape; scissors; Alka Seltzer tablets; water; metric tape measure or meter sticks

Preparation

Review and prepare materials. It is most important to use film canisters with lids that snap inside. Do not use lids that close around the outside of the canister.

In Class

This is an outdoor activity. If gusty winds are a problem, then place a quarter in the canister to keep the rocket from falling over. Launching near a wall where a metric tape has been hung or where meter sticks have been stacked may make it easier to judge how high the rocket goes. You may want to require students to wear safety glasses during this experiment as a general safety precaution. Everyone should stand away from loaded rockets when they are on the launch pad. It may take 15 to 20 seconds to build up enought pressure to launch, so a loaded rocket should not be approached prematurely. These rockets can shoot 5 meters or more into the air. No sharp objects should be placed on top of the nose cone or elsewhere on the rocket.

Wrap-up

One way to record the results of different "fuel" mixtures is to make a simple graph of height vs. amount of water. Such a graph gives a clear, visual record of the observations and can be used as evidence to support interpretations.

Extensions

Design and launch a rocket powered by two, three or more film canisters. Design a two-stage rocket. In each case, the students will need to work cooperatively to use the knowledge they've gained to solve problems of fuel mixtures and timing.

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