Edible Scale Instructor Materials

DESIGN FOR TOTAL RECYCLING

"THE INCREDIBLE EDIBLE SCALE"

INSTRUCTOR

OVERVIEW OF THIS PROJECT AND WHAT TO EXPECT

This project entails designing, building, testing, and consuming a scale that measures a weight between 5 and 30 grams. The first goal of the Incredible Edible Scale (IES) is to help students understand the principles used in constructing a scale. Second, because the IES can only be built with food, the students will learn to appreciate the difficulty of designing objects while considering the eventual disposal of the product.

SPECIFIC PURPOSE OF THIS PROJECT

The purpose of this project is to give students an elementary understanding of statics, and the properties of elastic materials while emphasizing disposability issues. Besides learning these basic principles, it is very important for the experience to be fun. This allows the students to relate engineering with the fun that it really is.

In design it is important to consider all of the criteria for a product. Often these criteria are in direct conflict. The student should come away from this project with understanding that in a engineering design problem, trade offs often need to be made between two or more attributes of a product.

THE REAL-WORLD PROBLEM RELATED TO THE PROJECT

We are all crew members on "Spaceship Earth". As such, we must eventually consume and/or recycle all of our products. This engineering design project is intended to focus on the significance of this recycling requirement when designing useful things.

Conventional weighing devices are simple to design and construct if there is no thought to eventual recycling. However, it becomes much more challenging when you must design the scale to weigh accurately as well as be completely recycled. This project gives an opportunity to explore this task. For today, this may sound strange; tomorrow designing for disposal may be routine.

SPACE AND TIME REQUIRED/DESIRABLE

Each student will require a clean table top area with dimensions approximately three feet by three feet. We suggest using clean plastic to cover any surface that can not be cleaned thoroughly. Both construction and testing of the scale can be conducted on the same area.

It should take an individual approximately two to three hours to build and test an IES. The IES should be tested directly after building because the mechanical properties of food change dramatically with time. The physics concepts needed to build an effective scale should take no more than an hour to demonstrate. We have found that this project can be repeated once with good results. Often, the students must first build an IES before they are able to grasp the principles needed to build a "good" IES.

MATERIALS AND EQUIPMENT NEEDED

The IES may be made only of materials which are sold locally for the purpose of consumption by humans. The tools used to manufacture the IES should be clean; we suggest using kitchen utensils such as knives, peelers, kitchen shears, drill bits, and other similar equipment. To calibrate the IES's, a laboratory scale or set of known weights is required; we have found that one scale to every 5 students is sufficient. If you do not posses enough scales or sets of weights it is possible to let each student have 6 nickels and let them know that each nickel weighs 5 grams.

We suggest that for younger students the teacher provides an assortment of food for the students. But for older students and younger students on their second round we suggest that the students be allowed to bring in the foods that he/she would like to use.

SUGGESTIONS REGARDING STUDENT TEAMS

Because the product must be eaten, we feel it is best for individual students to construct their own IES. You may want them to discuss and plan in small (2-3 person) groups but we recommend that each student make and test an IES.

PREPARING FOR THIS PROJECT - WHAT TO DO IN ADVANCE

Be sure to have adequate room, and enough utensils and scales for the number of students in your class. We feel it is best for the instructor to actually build and test an IES before introducing the project to the students. This will allow the instructor to help the students avoid some of the common problems associated with this task.

You will also need a "test object" for the students to make a weight estimate. We suggest you pick an object that weighs between 5 and 30 grams, and can fit in a cube 10 centimeters to a side (1 inch » 2.54 centimeters, 28 grams » 1 ounce). Things such as short pencils, 3.5 inch erasers, etc. make good test objects. Make sure that you are the only one who knows the true weight of the test object until all of the student estimates are given to you.

SUGGESTIONS FOR MANAGING THE PROJECT

Until they learn something about the principles involved in measuring weights, many students do not know how to begin this project. Therefore, we suggest showing how basic physics can be used to create a working scale.

Probably the easiest scale to make is based on balancing a beam. We suggest balancing a ruler or yard stick on a pencil as a demonstration. With this simple device, you can show students how to measure how many dimes equal the weight of two quarters. Then, knowing that a dime is approximately 2.4 grams you can estimate the weight in grams of the two quarters. You can also have the students answer questions such as, are 5 pennies heaver than 2 quarters, etc. To help you in this illustration, here are the approximate weights of coins:

• penny = 3.2 grams

• nickel = 5.0 grams

• dime = 2.4 grams

• quarter = 5.8 grams

To know when the balance is in equilibrium you should also have a reference next to the end of the beam. To demonstrate a spring-based scale, you can use a rubber band as described below. It is more difficult to build an Archimedes principle-based scale from food, but, it is a very accurate way of measuring weight. Therefore we would not discourage anyone from trying.

GUIDELINES FOR TESTING THE PRODUCTS

Once each student has completed the manufacturing stage, allow the student to weigh the test object immediately (we suggest limiting the time of their estimate to two minutes). The object must fit in a cube with 10 centimeters sides and weigh between 5 and 30 grams.

After making a confidential estimate (have them write their estimate down and give it to the teacher so as to not bias the other students' estimates) the student must then eat, including swallowing, the IES while being timed.

The Basic Performance Requirement for the IES is that it gives an estimate of the unknown weight, and that the student is able to consume the entire IES.

The Extra Performance Index (EPI) will be calculated by multiplying the amount of time (in seconds) it takes the student to eat the IES times the absolute value of the difference between the actual weight and the weight their scale estimates (in grams). The lowest index value represents the highest performance design. It is suggested that a minimal time of 240 seconds is assessed to the student even if he/she is able to consume the scale in less time.

EPI = |[[estimated weight(grams)]-[ actual weight (grams]]| X [eating time(seconds)]

IMPORTANT PRINCIPLES INVOLVED

There are at least three physics principles which can be employ in a scale (perhaps you or your students can think of others). One involves balancing two objects on opposite ends of a beam with the weight of one being known. A second is to use an elastic object (such as a spring or rubber band), allowing the weight to stretch or compress the elastic object and measuring the deflection. A third method is similar to measuring how far a boat sinks into a river when weight is added.

Balancing Principle

For the balancing principle, let's think of a see-saw. If a large person sits on one end of the beam, then in order to be in balance, a lighter person should sit farther away from the pivot point (Fig. 1). Realizing this, we may construct our scale in several ways. We may have several objects of known weight and place them an equal distance from a pivot point as the unknown

Figure 1. See-saw.

weight. Then, simply subtract or add known weights until the beam balances (Fig. 2). If the distances are equal then the unknown weight must equal the known weights.

Figure 2. Balance scale with equal distances.

Another way to construct it would be to have the unknown weight close to the pivot point and have a small weight be able to move along the beam (Fig. 3). The unknown weight is then calculated using the formula:

[unknown weight]X[distance 1] = [known small weight]X[ distance 2 ]

This is an expression of the principle of moment equilibrium.

You can calibrate this scale by noting the placement of the small weight with different known weights on the other side.

Elastic Principle

For the elastic principle, we only need to think of a rubber band on a hook. To calibrate this scale, you take different known weights and hang them on the rubber band, noting the distance that the rubber band stretches. You could also stack known weights on top of a spring and note the distance the spring compresses. Roughly, the distance stretched (or compressed) is directly proportional to the weight and doubling the weight would double the distance stretched (Fig 4). The equation can be written as:

known weight/ known stretch = unknown weight/unknown stretch

 Figure 4  Spring scale.

Archimedes Principle

The principle of using the depth a boat sinks into the water when loaded is called Archimedes Principle. Let's think of a glass of water with a "boat" floating on the top of the water (Fig. 5). If we note the height of the water on the "boat" when the "boat" is loaded with different known weights, we could have a very accurate scale.

With these principles of physics, your students can easily build a scale; the challenging task is doing it with foods that they like to eat and can eat quickly.

Figure 5. Archimedes scale.

PROVIDING SUCCESSFUL CLOSURE

In particular we encourage you in this project to emphasize the problems associated with disposal. All products must inevitably either be recycled or be designated as waste and therefore further crowd our landfills .

This project illustrates that engineering design requires the incorporation of knowledge from multi-disciplinary fields, conflicting objectives, and multiple goals. This is what makes engineering challenging; however, when students loose sight of these overall goals and begin to become obsessed with design details, their designs will become less successful. Even the brightest and most creative students can fall into this trap.

Most designs, even less successful ones, have strong points. Acknowledging those points along with some constructive criticism helps the students learn some of the more difficult points. We have found that group discussion following testing in which each team or individual critiques the strong and weak features of their own design is fruitful.

Finally, please make a special point of reminding students that this kind of creative activity is at the heart of engineering. If they found the project enjoyable, they should consider engineering as a profession.

SPECIAL SAFETY CONSIDERATIONS

First, it is very important that the students eat at their normal eating rate. This is to insure that they do not choke on their IES. Please be very careful while timing the students so that they don't get carried away with doing well and get a piece of their product stuck in their throat.

Cleanliness is important. While building the IES, be sure the students have adequately cleaned their hands and that they use only clean equipment to be sure the scale is fit for eating. The students may want to wash the test weight before putting it on their IES.

This project was developed by Michael I. Hessel, Jr.