Air Powered Rockets: Introduction


In this experiment we use an air powered rocket to demonstrate the conservation of momentum (mass x velocity). Namely, the total momentum of the air escaping the back of the rocket (a balloon) is equal in magnitude to the momentum gain by the rocket:

(mass of air) x (speed of air) = (mass of rocket) x (speed of rocket)

The concept of the conservation of momentum is introduced to the class with a demonstration involving the momentum balls. The class is asked to predict what will happen when one of the end balls is raised and then dropped against the other balls. The presenter tries this and the class observes the resulting motion of the balls. This motion can be explained using the conservation of momentum. When the ball that is falling hits the stationary balls, it comes to rest, and transfers its momentum to the other balls. Since all the balls are the same mass, the last ball in the chain moves with the same speed as the original falling ball. In this way, the total momentum of the system of balls is conserved mass multiplied by velocity remains constant. This concept will be further examined by the students using simple air powered rockets.

Students will construct an air powered rocket using a balloon attached to a fishing line guide wire. They will measure the distance the rocket flies as well as the time of the flight, allowing them to calculate the speed of the rocket. The students will be asked to repeat their measurements several times. This will give them some experience making quantitative measurements (as well as introducing them to the concept of measurement error). They will also be asked to combine their measurements to calculate the average speed of the rocket. The idea of average values may be new to the students, and if it is, this concept should be discussed. (The worksheet does not explain how to calculate the average speed).

Next the students will be asked to investigate the effect of adding weight to the rocket. They will increase the mass of the rocket by taping index cards to the side of the balloon and then measure the time and distance for several flights. Once again they will be asked to calculate the speed of the rocket. Since the mass of the rocket has been increased, the speed of the rocket should be smaller. However, the effect may be small and students may not observe a measurable difference in the rocket's speed.

Finally, they will investigate the effect of increasing the air resistance by attaching flaps to the rocket. The increased air resistance will further reduce the speed of the rocket. This illustrates the need to consider the presence of external forces when applying the law of conservation of momentum.

Of course, the conservation of momentum is simply a result of Newton's laws and this experiment could be used as a Newton's laws demonstration as well as one of conservation of momentum. In addition, the rocket demonstration is useful in discussing pressure. The mass of air exiting the balloon feels a force. That's why it goes. The force it feels is from the remainder of the gas inside the balloon which is at higher than atmospheric pressure.

Possible Discussion Topics

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