The Car with a Lot of Potential

Mechanical engineering applications—such as wind turbines, electric stoves and television screens—typically require the manipulation and transference of energy. This activity provides students with an example of the application of the elastic energy of a rubber band to a non-ideal mechanical setup, mimicking real-world challenges to design efficient engines with aerodynamic designs. Students test designs and tabulate observations based on varied experimental parameters.

After this activity, students should be able to:

• Test the effects of altering the number of turns of a rubber band on the motion of a model vehicle, reinforced with quantitative results.
• Test the effectiveness of varying one or more of the following on vehicle motion: wheel type, payload weight, rubber band type and lubrication.
• Report on the conclusions and collaborate with other groups to optimize vehicle parameters.

Wind-driven vehicle on a beach.CopyrightCopyright pikrepo.com 2020. Free for commercial use. https://www.pikrepo.com/nefoe/white-red-and-blue-sailboat-on-beach-during-daytime

Scientists and engineers rely on energy storage and conversion to make their devices work. For example, solar panels convert the sun's light into electricity. Using natural gasoline is an example of the conversion of chemical energy to mechanical energy in order to power vehicles to get people to the places they want to go. When rubber bands are stretched, the mechanical energy used to make the rubber band longer is converted to elastic potential energy in the rubber band. This energy is released when the rubber band is no longer stretched, or allowed to relax. This activity demonstrates the conversion of rubber's potential energy into mechanical motion with a simple LEGO model car.

This neat trick is no magic, but rather the mechanical engineer's ability to convert energy. Today, you learn how to convert such energy.

Background

Energy conversions are happening around us every day. Begin the activity by introducing common examples of the different types of energy; such as kinetic, potential, electrical, thermal, elastic, etc. For example, an electric heater converts electrical energy to thermal energy. A roller coaster car that starts at rest and drops converts potential energy to kinetic energy. An electrical stove is an example of the conversion from electrical to thermal energy. An electric car is an example of the conversion of electrical energy to kinetic energy. Many other different devices convert energy to work, such as computers, trains, space ships and electronic music players. Finally, an example of the conversion of elastic energy to kinetic energy is the use of a rubber band as the source.

A rubber band stores its energy by increasing its elastic potential energy when stretched or twisted (see Figure 4 to notice that when the gears are turned, the rubber is twisted, increasing the potential energy in the car). When the car releases, the potential energy is converted into motion, and you see the car move forward. To repeat the car movement, all you have to do is retwist the rubber band.

Before the Activity

• Gather materials and make copies of the Data Collection Worksheet (PDF).
• Construct a LEGO rubber band model car for each group, following the Vehicle Building Instructions (PDF).
• For each model car, make sure that the wheels are able to wind up the rubber band and that the potential energy is properly and fully released. It may help to lightly coat the rubber band in talcum powder to keep it from not sticking to itself upon winding.

With the Students

A teacher-guided experiment whereby students change their model car designs.

1. Inform students on the theory of the rubber band vehicle and its operation, as follows: "In a LEGO car, the elastic rubber band is the 'fuel' for the car. To pump 'fuel' into the car, you twist the rubber band, which adds elastic potential energy to the car's movement capability. When you release the car, the energy is converted to kinetic energy, making the car move forward."
2. Let students to experiment with the operation of the vehicle on the floor or at their desks, as shown in Figure 1.Figure 1. Students test their LEGO vehicle prior to starting the experiments.CopyrightCopyright 2010 Polytechnic Institute of NYU
3. With students, compare the rubber band to a car's engine. For example, with more gasoline (chemical energy), a regular car goes farther (mechanical energy). Similarly, with a rubber band tightly twisted (elastic potential energy), the LEGO car can go farther (mechanical energy). Given the same amount of gasoline, a real car with better engine efficiency than its counterpart goes farther. Similarly, given the same number of twists, a LEGO car with a good design goes farther than a car with a poor design.
4. Direct students to test the effect of changing their cars' designs by changing variables (shape, axle length, car length, wheel placement, etc.). See Figures 2 and 3 for a modified vehicle design. Mention that in life, engineers consider tradeoffs in designing cars. For example, suburban/passenger vans require more supporting material. More material means more weight and, on average, results in slower cars. While testing, suggest that students to use meter sticks or tape measures to evaluate the distance that their modified vehicles travel.
5. Tell students to be sure to twist their rubber bands the same amount every time so this becomes a valid measurement of the distance the car travels and an indicator of the car's performance.
6. Give students ideas for modifications:

• altered wheel configurations (Figure 2)
• lubrication (talcum powder) coating on the rubber band
• different types of rubber bands (engine)
• additional payload (Figure 3)Figure 2. Wheel configuration is one design variable that students may change to test its effect on vehicle performance.CopyrightCopyright 2010 Polytechnic Institute of NYUFigure 3. An example of how to alter a vehicle to accommodate payload by the addition of LEGO bricks (right). This is expected to ultimately affect the distance travelled by the car.CopyrightCopyright 2010 Polytechnic Institute of NYU

7. Remind students to consider the potential energy meter that is built into the vehicle when designing experiments. The potential meter indicates that more elastic potential energy exists in the car with more twists in the rubber band, as illustrated in Figure 4. Ideally, carry out each experiment under the same conditions (that is, the same potential energy or same number of rubber band twists). Students can use the meter to ensure that the rubber band is wound approximately the same number of times every time they conduct a test comparing car designs.Figure 4. Using the potential energy meter to verify rubber band turns are the same during vehicle redesign tests.CopyrightCopyright 2010 Polytechnic Institute of NYU
8. Tabulate the results of group test iterations and incorporate the findings from all groups into an "optimized" car to determine if the distance traveled by the vehicle is enhanced. For example, if students are familiar with gear ratios, the distance of the car can be compared against the gear ratio of the car design. Alternatively, students can compare the distance traveled of their car against the weight of the car. Encourage the groups to work with other groups to collectively optimize the vehicles performance based on their collected data.
9. As a class, discuss why certain modifications worked and others did not. Pick out keywords from the discussion and write them on the classroom board. Ask students to summarize the key findings from the optimized vehicles into paragraphs. After a few minutes, have a few students read their summary paragraphs to the class.

Pre-Activity Embedded Assessment

Prediction: Ask students to consider modern cars with respect to variables such as wheel type, lubrication, tires and engine. Ask students how they think changing these variables may improve or hinder a vehicle's performance. Ask them if they can think of other variables that can be changed. Discuss as a class.

Activity Embedded Assessment

Energy Storage Demonstration: Ask students to consider modern cars and what happens when you push the engine too far. Ask students to share their thoughts with the class. Then, in similar fashion, wind the rubber band car up until the point of failure. Ensure that the students witness the structural failures that ensue, reinforcing the point of energy storage limits of certain mechanical designs.

Ask students, what are the effects of altering the number of turns? How do changes in the designs of their cars affect the vehicles' motions? Restate the importance of experimental consistency and how this is established between sequential trials using the on-board potential energy gauge. Scientific experiments need to be repeatable, which is a guiding principle of the scientific method.

Post-Activity Assessment

Modification Keyword Summary: As a class, discuss why certain modifications worked and others did not. Pick out keywords from the discussion and write them on the classroom board. Ask students to summarize the key findings from the optimized vehicles into paragraphs. After a few minutes, have a few students read their summary paragraphs to the class. Compare student observations to gauge their depth of understanding of potential energy.

Graphing: As a class, graph the distance traveled at the different gear ratios for all the groups. 

Lightly coating the rubber band in talcum powder helps it not stick to itself upon winding.