Physics Lab Rockets

Joseph Dogadalski             Austin Community Academy H. S.  
                              231 N. Pine
                              Chicago, IL 60644 
                              312-287-5000

Objectives:

To promote interest in laboratory work at the beginning of the high school year.
To discover some qualitative variables involved in making a balloon rocket on a 
string.
To promote team activity and cooperation by preparing for and having a balloon 
rocket contest.
To shoot a model rocket outdoors and calculate its altitude.
Three levels of interest:
1. To calculate rocket's altitude by using angles, triangles, and proportions.
2. To compute its altitude by assuming its path is perpendicular to the eye 
level plane above grass and use the tangent function. 
3. To assume rocket's trajectory is a straight line at a non-right angle to the 
eye level plane and to have each team measure the angle of elevation at two 
distances in line from the launch point and move over to another line and do 
the same (see wire model).

Materials:

     FOR EACH GROUP: A picture of a line hanging from ceiling to lab desk with a 
straw and a balloon on it; roll of Scotch tape; straw; packet of a dozen 
assorted balloons; a monofilament line hanging from ceiling to be attached by 
each group to their lab station in the classroom; two sighting instrument kits 
made up of a ruler with a picture of an upside down protractor on it and a plumb 
line (string and a weight) taped to the point of vertices on the protractor. 
     FOR THE CLASS: A not too windy day; a grass field 30 by 100 meters; one or 
two identical model rockets ready to launch; half-dozen rocket engines that will 
propel the model to a height of 20 to 30 meters; three ropes with knots at 5 
meter intervals 30 to 40 meters long, ready to be stretched out at various 
convenient angles from the launch point; a coat-hanger wire model of a 
tetrahedron in which the vertices (corners) are held together by rolled up 
rubberbands and able therefore to be adjusted to various angles. 

Strategy:

INTRODUCTORY ACTIVITY: Blow up balloons and let them fly out of hand; watch 
their random motions; students will thread the prepared line from the ceiling 
through a straw and tie it to the lab top; tape blown up balloon(s) to the straw 
and let them fly; 
     While the groups are experimenting, the teacher can walk around and give 
clues to "What's happening?".  Discuss: action-reaction, conservation of 
momentum, pressure in an uninflated and inflated balloon, catastrophic punctured 
balloon, and the conditions needed to make a balloon rocket.  Also the teacher 
will walk around with the wire tetrahedron and show the possible flight paths of 
the model rocket to be fired, the geometric line of sight, and the angle of 
elevation to be measured. 
     Students are to be preparing their design for the contest "shoot off" at 
the "teacher's shooting range".  Maximum height from floor wins.  After 
experience with a half-dozen shots, students should be ready for the contest. 
     BALLOON CONTEST DATA (best of two shots)
     Trial                         1           2         (height from floor) 
     Team A,B,C,...(circle one) 

ACTIVITY: After the contest the whole class will go out to the grass field to 
shoot the model rocket.  Before the rocket launch, student groups will learn the 
use of their sighting instruments.  Before each firing, teams will pick one 
station on the stretched out ropes radiating from the point of launch.  If a 
student group wants to launch the model rocket, they must learn the 14 safety 
points of the model rocket society which come with the engines.  Otherwise the 
teacher should always launch the rocket. 
                                                              
MEASUREMENTS
   TEAM A,B,C,...    Distance of station from          Angle of elevation
   (Circle one)      launch point on line 1,2,3.
   First launching
   Second  "
   Third   "
   ...

Conclusions:

     Rockets should reach a consistent altitude by using the same model, same 
engines, and the same launch angle.  Students will learn the safety points of 
launching a model rocket. 
     On the first level of interest, students, by drawing similar triangles with
the measured angle of elevation, should be able to calculate the altitude by 
making a proportion. 
     On the second level, students should determine the altitude by making the 
tangent of the elevation angle equal to unknown altitude divided by the distance 
from launch point and solve the equation. 
     On the third level, a group will divide into two and stand in two locations 
on one of the rays (knotted ropes).  They will measure two elevation angles and 
draw an oblique triangle the base side being the distance between their 
positions on the ray.  Having two angles and the included side of an oblique 
triangle, the group can determine a second short side by the "Law of sines".  
Using this side, the closer elevation angle and the sine function, students will 
be able to figure out the altitude of a rocket with any non-right angle 
trajectory to the level plane.  In all these cases, a good figure is worth more 
than all these words; but especially the wire tetrahedron model will be useful 
in showing the various possible trajectories in three dimensions. 
     A measurement of time (seconds) opens up the whole world of motion.

Evaluation:

     Student grades will be based on scale drawings of triangles, angles of 
elevation and the appropriate calculations.  Also, the answers to the questions 
below will be considered. 

Questions:
1. When does a balloon act like a rocket?
2. Name some variables that influence the balloon's flight?
3. 20 is to 40 as 60 is    .
4. Similar triangles have the same shape but a different    .
5. Would a real rocket work better in outer space?
6. A real rocket's mass is mostly     .