Jeffrey C. LaCombe,
Materials Science and Engineering Department,
Rensselaer Polytechnic Institute, Troy, NY 12180
 
 

The concept of a portable classroom drop tower was first popularized by (if not introduced by) NASA’s Lewis Research Center. The Microgravity Demonstrator apparatus, described by Rogers and Wargo [1] is a portable and compact demonstration kit that fits entirely into a trunk—television and all. While of excellent design, the NASA hardware is largely custom-machined, is expensive, and in short supply. Rogers and Wargo offer some basic advice on building your own drop tower, which can be found in the above referenced description of the Microgravity Demonstrator.

The design described here picks up where the original NASA design leaves off by providing a portable, yet inexpensive and easy to construct apparatus. Some schools have televisions and video tape recorders already available, allowing costs for the drop tower to be limited to as little as $200 to $300 (for parts) depending on the choice of the video camera.

When an object freely accelerates in a gravitational field, it is said to be in a state of "free fall" and experiences approximate "weightlessness". When the free fall condition is produced in a drop tower, the acceleration levels approach 1/100 – 1/1000 of earth’s gravity, which for many purposes (including classroom demonstrations) can be though of as "zero-gravity". This "weightless" condition enables a large range of useful experiments and classroom demonstrations of important concepts. Particularly well suited, are numerous demonstrations of the fundamentals of mechanics. These can easily be extended to cover subjects such as natural convection, and fluid mechanics. Some classroom applications of the drop tower can also involve student-designed experiments, which can explore physical principles as well as the "Scientific Method" and basic laboratory techniques.

This 2-meter drop tower provides approximately 0.6 seconds of reduced gravity. This is enough time to conduct and observe a wide variety of demonstrations, especially if the demonstration is recorded on videotape. The hardware described here consists of a support tower for raising the drop box, the drop box itself, with a small mounted video camera, the experiment "modules", a catch basin, and video equipment to allow close study of the demonstration via slow-motion playback.

The construction of the drop tower (Figure 1) is possible with hand tools, though access to a table saw and a drill press (available at many schools) will facilitate the process. The details of the construction dimensions are not critical to the functionality of a drop tower, so discussion of the construction will be focused on more broad issues. A list of the major items that will be needed is given in Table 1. 


 

Support Tower
The support tower primarily consists of PVC pipe and PVC pipe fittings. The vertical supports are constructed in three sections, joined by PVC couplings, allowing easy disassembly and compact storage. Elbow fittings attach the top cross member to the vertical sections. The PVC pipe is lightweight and provides adequate rigidity for the structure. One difficulty with off-the-shelf PVC fittings is their tapered interior dimensions, which make them more suitable for permanent assembly. If a coarse grit sanding drum is used to level off the taper on one side of each coupling (or elbow), a better slip fit can be obtained. By leaving the taper on the one side of each fitting, and attaching this end permanently to one end of each length of PVC pipe using PVC cement, a stacked, sectional tower structure results (see Figure 2). To this PVC structure is attached the pulleys and cleats that manage the utility cord that lifts the drop tower.

Drop Box
The drop box is a 4-sided plywood container (see Figure 3) with a video camera attached to one inside wall, providing a view of the box interior. Slots are incorporated into the box to allow individual experiment modules to be slid into and out of place for a variety of demonstrations. On the top exterior surface are eye-hooks for attachment of the cord that is used to raise the box to the top of the support tower for dropping. An acrylic shield may also be placed in front of the camera to protect it against any dislodged parts that may result from the impact at the end of the drop. Typically, this is not a problem, though experience shows that student-designed experiments can have unexpected results, and if left unprotected, flying experiment pieces have the potential of striking the camera and doing damage. As an option, a stopwatch can be placed in the camera’s field of view to allow precise timing of experiments as needed.

Experiment Modules
The drop tower described here includes five experiment modules. Each of these is permanently mounted onto an appropriately sized plywood platform that can quickly slide into slots in the drop box (Figure 3). This feature quickly places pre-fabricated experiments correctly in the field of view of the camera, and thus streamlines classroom use. These pre-fabricated experiment modules will be described more below.

In addition to the pre-fabricated modules, empty modules (plywood only) are included with the drop tower package. These modules, or even similarly sized pieces of cardboard allow an instructor to devise their own demonstrations or even oversee student designed and constructed experiments. These experiments can be temporarily mounted on these modules and placed in the drop box similarly to the pre-fabricated "standard" experiments.
 

The Drop Tower Microgravity Demonstrator can be used to demonstrate a wide variety of physical principles. Several of these experiments can be kept with the drop tower for easy demonstrations. Some of the experiments described below were inspired by the work described by Rogers and Wargo, while others were developed by a team of educators during the design phase of this drop tower.

Ring Magnets: The principles governing the balance of forces, can easily be demonstrated via a variety of experiments in a drop tower. The version depicted in Figure 4 involves three ring magnets constrained to slide along a dowel, capped by two end pieces. Under vertically oriented, stationary conditions, the force of magnetic repulsion between the magnets is counter-acted by the weight of the magnets, resulting in the rings "settling" at the bottom. However, under free fall conditions, the magnets push each other as far away as they can, constrained only by the ends of the dowel. When the magnets are dropped while being recorded by the camera and VTR, the rapid shift in magnet positions is later easily observed.

The Kitchen Scale: A common kitchen scale can be used to demonstrate the difference between mass and weight as well as inertia and how a spring-scale works. When a mass is placed on the scale, it compresses an internal spring, which lowers the needle to readout position "A" in Figure 5. This is the weight of the mass under earth’s gravity. When this is allowed to free-fall using the drop tower, students may initially expect the needle to jump to the "zero" mark on the readout. What actually happens is the needle will be "pegged" at the top end of the scale since the energy stored in the compressed internal spring is suddenly "released" as kinetic energy (motion). The inertia in this motion carries the needle to the pegged position at "B". However, the needle does not stay here. It will instead settle at position "C", which is the relaxed position of the scale’s internal spring. Note that this too will not be at the zero mark on the scale’s readout, since the self-weight of the scale parts are normally calibrated out of the reading. All told, the common kitchen scale can be used in coordination with a drop tower to demonstrate a range of physical principles that may not be immediately obvious.

Droplets: Another interesting demonstration of the consequences of mechanics involves two immiscible liquids of different densities. A suitable demonstration of this type is often readily available in "science stores" or as "executive toys". The liquids (commonly colored oils) are contained in an hour glass-type arrangement, with the more dense liquid allowed to drip through the less dense liquid, and fall slowly to pool at the bottom, as sand would fall through air to the bottom of an hour glass (see Figure 6). The difference in this case being the droplets of one liquid falling through the second liquid in a slow, easily recognized manner. These falling droplets are seen to come to a complete standstill as gravity is effectively removed when the whole fixture is in free-fall.
 

The Candle Box: The principles of buoyancy, diffusion, and natural convection are all important to process of combustion. As a flame burns, it creates temperature gradients in the surrounding gas (air) and thus buoyancy driven natural convection is created. This convective motion dramatically enhances the delivery of oxygen to the flame, and subsequently enhances the burning process, and gives the candle flame its recognizable shape. Under the free-fall conditions created by a classroom drop tower, a candle flame is seen to dramatically decrease in visible intensity in comparison to the stationary (regular gravity) condition. This results from the significant reduction of convective transport of oxygen to the flame, leaving the slower molecular diffusion process to deliver oxygen to the flame. In addition to the change in the flame’s intensity, the shape is also affected. Under free fall conditions, the flame forms a sphere-like shape, rather than the conventional "flame-like" shape.

Construction of a candle box demonstration should utilize only flame-resistant materials. Good designs feature an enclosure to control the "draft" created by rushing air during free-fall. The enclosure can readily be made using an aluminum "project box" available at electronics shops (Radio Shack) with a window (glass) and sufficient ventilation to allow air in, and combustion products out. Other variations will work, though it is important to have the proper amount of containment. Too little ventilation will starve and snuff the flame, while too much will introduce convection due to rushing air during free-fall. A trial and error process may be necessary.

The Battery Tester: Most educators have probably experienced the classroom phenomena of students expecting something to happen simply because the teacher is taking the time to do a demonstration. To address this, a null-effect demonstration can be included where there is no measurable difference in the experiment under normal and free-fall conditions. The inexpensive version constructed here is simply a needle-gauge battery tester with a fresh battery, securely fastened together. A drop in the drop tower shows no movement of the battery gauge’s needle, indicating that gravity doesn’t affect the voltage of the battery.

Student designed experiments: An effective way of including a drop tower into the classroom is to guide students through the process of coming up with an idea for an experiment or demonstration, devising how to execute it, perform the experiment, and then learn from it. This can be a strong introduction to the scientific method, but requires guidance from the instructor, to encourage students to take more than a trial-and-error approach. It could be expected that excited students will want to drop everything from the classroom hamster to water balloons. By proposing experiments to the instructor, feedback can be given to guide the student in thinking about what they might expect to see, and what the teacher accepts as a "proper" experiment.
 

The drop tower design presented here has the advantages of being affordable, portable, and relatively easy to construct using common tools and supplies. Note that it is not necessary to utilize a tower structure and catch basin. Instead, a pulley can easily be mounted directly into many classroom ceilings. Such a design would be largely simplified, and easier to construct, though portability would be reduced.

A classroom drop tower can be an effective teaching tool which, when coupled with appropriate discussion and exercises, has the ability to leave a lasting impression on students. In addition to instructor-operated demonstrations, a drop tower makes an interesting laboratory exercise, where students can devise, construct, and execute their own experiments, thereby independently exploring physical science with the instructor’s guidance.

When constructing your own drop tower, the best place to start is the NASA pulication by Rogers and Wargo [1]. This publication is available through many NASA resources. The many Teacher Resource Centers such as the one at NASA Lewis Research Center (216 433-2017) are good places to start when looking for NASA educational documents. While there, the Microgravity Teachers Guide [2] is also an excellent resource and can be obtained via similar channels. Other suggested resources are provided in Table 2, which lists places you might start with in a search for information and components.

The author wishes to thank NASA’s, Microgravity Research Program Office at Marshall Space Flight Center, the Graduate Student Research Program, and the Microgravity Science Division at Lewis Research Center, (especially Fred Kohl, and Terri Rodgers) for assistance and financial support. I also would like to thank the Isothermal Dendritic Growth Experiment (IDGE) team and the participants of the 1997 Microgravity Summer Institute for discussions and support in the design and construction of the Drop Tower Microgravity Demonstrator.