Author: Tess Ewart
Students will be challenged to build a shock absorbing structure using different polymeric materials or rubber that would best protect a gelatin "head" during an impact.
What should students know as a result of this lesson?
What should the students be able to do as a result of this lesson?
Gelatin (depending on the abilities of the group, you may choose to make the gelatin "head" using the flavored, sweetened gelatin dessert preparation or the finger gelatin (sometimes called jigglers) preparation according to the box directions. See additional considerations.
Optional = Gelatin Molds - such as egg or helmet molds (you will need at least one gelatin "head" per lab group - more if you want to give the students additional trials, see additional considerations)
Assorted polymeric and rubber materials (rubber bands, plastic bags, foam, spray foam, Styrofoam [packing peanuts, floral foam, cooler, bean bag fill], soles & linings of shoes, plastic bottles, bubble wrap, racquet & tennis balls, buckets, loose rubber surfaces from playground, etc.)
Various materials to assist in construction (tape, scissors, popsicle sticks, etc.)
This activity is designed to be a culminating activity after students have discussed forces and energy conversions during class. You may want to review that material before continuing.
Ask students, "What sort of materials or structures protect objects that are falling or are going to crash?" (Accept all answers such as helmets, air bags, elbow/knee pads, pillows, parachutes, etc.) "How do these materials protect the objects that are falling or are going to crash?" (Lead students toward the understanding that these materials absorb the shock [forces and kinetic energy] the object experiences)
You may show the students our video of what happens without shock absorbers below:
Give students their challenge, "design and build a shock absorbing structure using the various polymeric and rubber materials available that will protect the gelatin "head" from a drop of 2 meters. Drop an unprotected gelatin head from 2 meters to show students what will happen to the head if there were no protection for it.
Give the students a copy of the scoring rubric to help guide their thinking - note, for example, that thinner landing pads receive more points than thinker ones. Have students brainstorm ideas for their protective structure and make a drawing/diagram of the structure they are going to use. If students want to use items that the teacher has not provided, the students will need to provide these. Be sure to monitor their choices for safety. Have a discussion on proper safety procedures to use when building their structures.
Assessment: Monitor students' ideas for protective structures to be sure everyone understands the goals of the lesson.
Have students construct their protective structures and test how well they protect the gelatin head. Make sure that students are employing safe practices as they do the construction and testing. Monitor each group to ensure they are dropping their structure from the 2 meter height. Depending on the time available to the teacher, give students a chance to alter the design of their structure after one drop to see if they can improve on their design.
Assessment: Successful drops of the gelatin heads, points should be awarded for neatness of construction, a successful drop and the thickness of the material used to protect the gelatin head. Measure the thickness of the structure at its widest point. The thinner the structure, the more points awarded. (See suggested rubric)
Students should report findings (structure design & materials vs. ability to protect gelatin head). Students should discuss the characteristics of the successful structures and the characteristics of the unsuccessful structures (lots of air space between material, softer material, etc.).
Ask the students, "what forces are acting on the gelatin head?" (Gravity, friction due to air resistance, force of impact with ground) "What types of energy does the gelatin head experience?" (Gravitational Potential Energy at the release point converting to Kinetic Energy as the gelatin head falls - See Content Knowledge) What happened to the impact force and kinetic energy acting on the gelatin head with the successful structures? (they were dissipated from the gelatin head) Why is it important to wear safety equipment to absorb shocks during different activities? (to dissipate the forces or energy acting on us to help prevent injuries)
Assessment: Listen to students' discussion of successful structures to judge if their reports are supported by the findings that you observed as experiments were being conducted. Monitor their understanding of the physical forces displayed in this activity by noting the accuracy of students' answers to oral questions.
Ask students, "what is a polymer?" (see Content Knowledge) Polymers can provide a lightweight, strong, shock absorbing material. "What is rubber comprised of?" (see Content Knowledge) Determine which groups used mainly plastic and mainly rubber for their structures (Teacher will have to identify for the students each material as being either a plastic or rubber).
Assessment: Give students 2 - 3 minutes to complete a brief written summary of the findings. The prompt will be, "Which polymer makes a better shock absorber based on our experiments - plastic or rubber? Justify your reasoning."
This activity is designed to be used after instruction on forces that affect an object and kinetic energy.
Common Core Standards:
Shock absorbers are designed to absorb or dissipate the forces and kinetic energy acting on a structure. Some ways these devices may absorb the energy is to utilize elastic materials, air, fluid or springs.
Potential energy is stored energy due to an objects position or chemical bonds. There are different types of potential energy. Gravitational potential energy is due to an object's height above ground. Elastic potential energy can be found in objects such as a stretched rubber band. Elastic materials, such as a spring, trampoline or a rubber band, resist being stretched out of shape. The different types of potential energy can be converted to kinetic energy. An example of this would be when a bungee jumper jumps off of a bridge. The gravitational potential energy due to the person's height is converted into kinetic energy during the fall. Kinetic energy is the energy of motion. The sum of potential and kinetic energies make up an object's mechanical energy. The weight of an object is the measure of gravity acting on an object.
The simplest definition of a polymer is something made of many units. The units, or monomers, are made up mostly of carbon and hydrogen atoms and are linked together to make a chain from at least 1000 atoms to many 1000's of atoms in a row. While some polymers, like polyvinyl acetate (Elmer's Glue) can exist in the liquid state at room temperature, they can be made more solid or gel-like by the addition of a crosslinker. A crosslinker is a small molecule or ion which bonds to two different polymer strands and restricts the movement of the individual polymer strands. This causes the characteristics of the polymer to become more gel-like and less fluid. Since a new substance is formed by the addition of the crosslinker, a chemical change has taken place. (C. Helfer, The University of Akron)
Scientists continue to develop cheaper, stronger, and better polymers. One method of accomplishing this is through compounding, or the addition of a variety of materials to a polymer during the manufacturing process. The goal is to make the best product possible and assure trouble-free manufacturing of the product. Additives change the properties and/or improve the ease of processing the polymer. The final product performs better and can be offered at lower cost to the consumer. Additives may be fillers (extending agents), reinforcers, antioxidants, heat and ultraviolet stabilizers, flame retardants, plasticizers, colorants, lubricants, foaming agents, and antistatic agents. (K. Owens, The University of Akron)
"Originally, a natural or tree rubber, which is a hydrocarbon polymer of isoprene units. With the development of synthetic rubbers having some rubbery characteristics but differing in chemical structure as well as properties, a more general designation was needed to cover both natural and synthetic rubbers. The term elastomer, a contraction of the words elastic and polymer, was introduced, and defined as a substance that can be stretched at room temperature to at least twice its original length and, after having been stretched and the stress removed, returns with force to approximately its original length in a short time. Three requirements must be met for rubbery properties to be present in both natural and synthetic rubbers: long threadlike molecules, flexibility in the molecular chain to allow flexing and coiling, and some mechanical or chemical bonds between molecules." (Partridge, "Rubber", in AccessScience@McGraw-Hill)
Rubber materials acts as a shock absorber by "arrest[ing] a moving object with minimum load transmission. They act by decelerating a moving mass with a resistive force, ..." (p.85, Engineering with Rubber, 2nd Ed., Alan Gent). Imagine a hammer striking a solid rubber block, the hammer is stopped (decelerated) by absorbing the force of the impact and dissipating that force over the block of rubber, without transferring the shock/impact force to whatever is holding the rubber block. The rubber acts like a spring. (C. Laursen, The University of Akron)
"Frequently a spring in the form of a block of very elastic material such as rubber absorbs shock in a mechanism. For example, the four legs of a punch press rest on four blocks of rubber. The rubber pads prevent the die-closing inertia forces of the press from transferring down through the legs to the floor with impact or hammer blow proportions. With the rubber pads under the press legs, the force on the floor builds up relatively slowly and no shock is evident. As the acceleration of the die block goes to zero, the inertia force goes to zero and the rubber pad springs, which were deflected by the press blow, are relaxed and ready for the next stroke of the press die. The whole press moves up and down relative to the floor, but by proper selection of the rubber pad the elastic constant is such that this motion is small." (L. Sigfred Linderoth, Jr., "Spring (machines)", in AccessScience@McGraw-Hill)
Students need to use caution when using building materials (scissors, glue, etc.) during construction. Use eye protection and aprons when working with any foaming materials.
An effort should be made to recycle any materials that are accepted at recycling centers.
Many industries incorporate shock absorbing structures such as construction (earthquake proofing buildings), medical (prosthetics), transportation (shock absorbers on the wheels or suspension, airbags, aircraft landing gear) and sports (helmets, protective gear).
Have students research how shock absorbing or dampening structures are used in different industries such as technology (mobile CD players, laptop computers, personal DVD players), construction (earthquake proofing buildings), medical (prosthetics), transportation (shock absorbers on the wheels or suspension, airbags, aircraft landing gear) and sports (helmets, protective gear).
Grouping Suggestions: Care should be taken when choosing groups so that student abilities, gender and ethnic backgrounds are considered.
Pacing/Suggested Time: Teachers will need to prepare the gelatin before class. Gelatin can be prepared in a rectangular pan and cut up into squares or made using optional gelatin molds. We recommend reducing the amount of water per box by 1/3 to give a more robust "head" than would result if using the recommended amount for the dessert recipe. You can get two different sizes of molds in the shape of a brain at http://www.yankeehalloween.com/qwiggle.html , egg and helmet molds at http://www.kraftcornerstore.com, or at stores such as Target or Wal-Mart.
Time frame: it will probably take 1 day to do the engagement/brainstorm activity, 1 day to create the structures, and 1 - 2 days to test. You may want to give the students some extra time between the brainstorming activity and the exploration/construction activity to refine their designs or to bring in extra building materials.