# Gravity Cruiser

Student teams design and construct a vehicle that is powered by gravity. A weighted lever connected to an axle by string rotates on its fulcrum; as the weight descends it causes the axle attached to the string to rotate, propelling the cruiser forward. Concepts explored include potential and kinetic energy, friction, inertia, momentum, diameter, circumference, measurement, graphing, and constructing a prototype.

The gravity cruiser is a prototype toy presented by the fictitious toy company EarthToy Designs. The engineering and design of the toy is incomplete, and student design teams are asked to complete the design process. These design teams provide many of the services required for the toy product to move to the next stage of development. The teams will do product testing and engineering and create their own cruiser designs.

In this challenge, students will focus on understanding the relationships between the "sweep" of the lever arm, the number of winds the string makes about the axle, and the distance the gravity cruiser travels. They will also investigate how the diameter of the wheels, the diameter of the axles, and the amount of weight placed on the lever affect the gravity cruiser's speed and distance. The interplay of these factors is not simple—it offers a rich challenge in critical thinking while at the same time providing an enjoyable "vehicle" for learning how to use the experimental method to test hypotheses and solve a tricky engineering problem.

After learning how to control the performance of this toy through a series of controlled tests, student design teams create their own customized toys. The set of toys that students design will be presented at the end of the challenge.

#### Lesson Plan Overview

Lesson 1: Introducing the Gravity Cruiser (45 min)—Students receive a letter from a fictitious toy company, EarthToy Designs, inviting them to create their own gravity cruiser toy design based on specifications provided by the company. Students discuss the requirements described in the letter. They are introduced to the engineering design process and the scope of this design challenge: to build a gravity cruiser, figure out how it works in order to predict its behavior, and use this knowledge to design their own customized gravity cruisers that meet specific performance goals.

Lesson 2: Building and Testing a Gravity Cruiser Chassis (45-90 min)—Students are organized into teams. Using only the given materials and no direct instruction, teams build a gravity cruiser chassis that rolls freely.

Lesson 3: Designing and Building the Lever and Tower (120-180 min)—Students design and build an arrangement for powering the model. This involves creating a support structure and a drive mechanism, which is a lever arm. They must also decide how to attach a string to the lever arm and wrap it around one of the axles of the chassis.

Lesson 4: Troubleshooting the Drive Mechanism (90 min)—After the preliminary testing done in Activity 3, students systematically troubleshoot their designs, working toward an overall design that performs consistently. Adjustments may need to be made to strengthen the tower and to make sure that the lever arm moves freely with minimal friction.

Lesson 5: Systematic Testing: Wheel Size (45 min)—Teams use their gravity cruisers to conduct formal investigations into the effects of wheel size on vehicle performance. To investigate the effects of wheel diameter, students track their cruisers' travel distance as they attach different-sized wheels. Students log their experimental data and find that larger wheels allow the gravity cruiser to travel a greater distance.

Lesson 6: Systematic Testing: The Lever Arm (45-90 min)—Students examine how the lever design affects the behavior of the gravity cruiser. Teams follow a procedure similar to the one they used in Activity 5; they systematically change one dimension or feature of their gravity cruisers; test the cruisers' resulting distance and speed; and log their results. When investigating the lever dimensions, teams focus on the following:

• the distance from the fulcrum to the string attachment
• the distance from the fulcrum to the weights
• the amount of attached pennies.

Lesson 7: Systematic Testing: Axle Diameters (90 min)—Students examine how the axle diameter affects the behavior of the gravity cruiser. Teams follow a procedure similar to the one they used in Activity 5, systematically changing one dimension or feature of their gravity cruisers; testing the cruisers' resulting distance and speed; and logging their results.

Lesson 8: Designing Gravity Cruisers to Meet Criteria (45 min)—Teams are challenged to modify their gravity cruisers to optimize the distance the cruiser can travel. Using the class graphs and data tables collected in earlier activities, teams determine the best combination of wheel size, axle circumference, weight, and lever dimensions to produce the desired vehicle performance. Finally, each team prepares a set of design specifications for the gravity cruiser it will present at the end of this challenge.

Lesson 9: Building and Testing Gravity Cruisers (45 min)—Using the gravity cruiser design specifications they developed in the previous activity, design teams configure their vehicles to meet these specifications. Teams conduct a series of test runs and record the performance results produced by their modifications. Teams also begin to plan their presentations of their cruiser prototypes.

Lesson 10: Presenting the Gravity Cruiser Designs (90 min)—In this activity, student design teams share their final gravity cruiser designs with the class and, if possible, invited guests. Teams present their Gravity Cruiser Design Specifications, Reproducible Master 14, and test results and then describe the specific performance features of their particular design. Each team demonstrates its gravity cruiser. Members of the audience may then ask questions of any team member concerning the operation of the vehicle. The class discusses the relationship between the design of the cruisers and their performance. Students then reflect on how their understanding of the gravity cruiser has grown since they began the challenge.

#### Kit Materials

 Item Quantity Pre-scored chassis platform sheets 14 Corrugated plastic strips (2"x24") 91 Wheels -  large 48 Wheels -  medium 56 Wheels -  small 48 Wooden dowels 56 Plastic  straws 56 Penny vials 24 Small (19-millimeter) binder clips 144 #32 rubber bands 50 Brass finish round-head fasteners 300 Boxes of small paper clips 200 Roll of kite string 1 Teacher manual on CD 1

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