Robert W. Mellette
Topic
Micro-rocketry
Rationale
This simple desk top activity can easily be carried cut in the classroom. This exercise is an effective and practical way to investigate Newton’s Third Law of Motion, without having to purchase expensive equipment or relying on fair weather required for launching model rockets.
Objective
To demonstrate in a safe and simple way Newton’s Third Law of Motion.
Materials Required
shallow plate or dish
waxed stencil paper
dropper pipets
liquid dishwashing detergent
single hole puncher
scissors
pens or pencils
figure available in print form
Procedure
It is best to have students work in pairs for this activity. The design requirements for this project are very simple. The micro-rockets should be approximately one inch (2.54 cm.) in length. The front end should be pointed to form a nose cone, so that the “‘rocket” will move easily over the water. The width of the rocket is determined by the number of combustion chambers. The rockets shown here vary from one-half inch to about one inch in width.
Using a hole puncher, punch a hole approximately one-half inch from the “‘aft”’ or rear end. This hole forms the “‘combustion chamber”’. A narrow nozzle is cut from the aft end to the combustion chamber. The dimensions of this nozzle is not critical, however, its width should be no more than half the width of the combustion chamber, and it should taper outwards toward the aft end as it leaves the combustion chamber.
Students may elect to design their own micro-rockets or may wish to follow the examples shown here. The examples A, B, C and D pictured on the diagrams can be photocopied and printed directly onto stencil paper for student use.
After the students have constructed their microrockets, direct them to fill their plate or shallow dish with clean, fresh, cold tap water. Their rocket should be placed on top of the water at one edge of the dish, with the nose cone pointing toward the center of the dish. Using an eyedropper, place one drop of liquid detergent “‘fuel”’ into the combustion chamber. The micro-rocket will zoom across the water’s surface. Challenge students to carry out several investigations using this simple example of action-reaction forces. Encourage students to make and then test their predictions as to the path or course that the micro-rockets A, B, C and D will follow. What will happen if equal amounts of detergent are placed into both combustion chambers of rocket A simultaneously? What will happen if only the right or left combustion chamber is fueled? What will be the likely path or course followed by rockets B, C and D?
Background Information
This simple and elementary experiment, which is
easily performed in the classroom,has real and meaningful lessons for the students involved. Newton’s Third Law of Motion which states that for every action there is an opposite and equal reaction is visibly evident to the student. The action of the escaping liquid detergent out the aft nozzle causing the forward movement or reaction of the rocket across the surface of the water. Through these simple investigations students can be led to see the same forces at work on all rocket propulsion systems. In the case of the Space Shuttle, the experiments with the micro-rocket with two identical combustion chambers is very useful in having students make important understandings as to the critical importance of the two solid rocket boosters (SRB’S) generating identical thrust for stable controlled flight of the Shuttle.
It should be explained to students, if they do not ask, that one of the physical properties of a soapy liquid introduced into clean water is a tendency to spread out evenly over the entire surface of the water. This movement of the soapy liquid rearward out of the aft nozzle causes the forward thrust of the micro-rocket. Unfortunately, the soap film will,in time, spread over the entire surface of the water and the micro-rocket will cease to respond to fresh drops of “fuel”’. To obtain the best results instruct the students to change the water after each run.
