# Air Powered Rockets: Introduction

## 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 Ñ 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

- Analogy between the medicine ball demonstration and the rocket experiments
- Importance of external forces
- friction in the medicine ball demonstration
- air resistance for the rocket

- Applications to real-world things (see supplemental material)
- jet airplane
- space shuttle / rocket

- Idea of average values
- Measurement techniques and errors