An aircart design was developed by the Hands On Physics project as a primary object for students to build as they learn about motion. HOP is a curriculum in which students construct their understanding of physics as they build their own equipment, using inexpensive electronic components and readily accessible materials. It stresses teamwork as well as individual responsibility for learning.

Introduction

Dragging heavy objects over rough ground or choppy water is an age-old drudgery. Humans have invented many ways to make this simple process easier. We invented the wheel, refined it, and used it in many clever ways. Even more amazing is the fact that we created vehicles which fly! Vehicles which travel on land may be moved forward by pushing backward on the surface of the roadway and boats may be propelled by pushing backward on the water they float in. To fly, what can you push against? The spinning "air screw" or propeller was developed to push against the obvious - air. Not only did this technical innovation allow engine-powered flight, but it provided another possibility for moving terrestrial vehicles as well. Test this possibility with a model.

Performance Criteria

This aircart must use a propeller powered by no more than 3 volts.

You may want to maximize its top speed, its ability to speed up, or its load carrying capability. There are other possibilities.

Tools

• Two dangerous tools are required for this project: a very sharp knife and a superhot soldering iron. Either of these tools can cause painful injury, so be alert and use them with care. A small knife with a segmented blade works well for cutting cardboard, and a soldering iron and solder for making good electrical connections.

• A hot glue gun allows quick assembly. Because the hot glue cools quickly, you must be confident about the placement of glued parts and you must work rather rapidly.

• A ruler is necessary for measuring and for use as a straight edge.

• Although a compass is designed for drawing circles, it has a sharp point which also makes a good hole punch. Of course you can also use it to draw circles.

• Use a work-board for cutting and gluing. If a work-board is not available, use pieces of scrap cardboard. Do NOT cut directly on your desk or workbench.

Cardboard: corrugated cardboard is a good material for building small structures. Parts & Pieces:

Radio Shack™ 800-THESHACK
275-409 slide switch (2/pk)
270-324 battery clips (9v)
270-382a AA battery holder
273-223 DC Motor

Local Hobby Shop
Cox #5d4p propeller, 5" black plastic
Mangelsen's #25173 wood wheels 3/4" x 3/16"
AA batterieS
pop rivets, aluminum 1/8" diam, 1/4" grip

Construction

The design presented here has been tested and works well. (Fig. 1) The strength and rigidity of this aircart depends on several triangular structures. During construction it is important to maintain equal sides and 60 degree angles so things will fit together nicely.

1. Base (Fig. 2): The base may be cut from an 18cm x 28cm piece of corrugated cardboard. The aircart is build up from this base; wheels attach to it and the motor is mounted on it. Cut out the base, and score the folds. Fold the sides to make triangular beams. The flaps should meet in the center and cover the front wheel hole. Mark this hole on the flaps and cut out the semicircles. Glue carefully, the angles must be 60 degrees or the aircart sides won't fit right.

2. Sides (Fig. 3): The aircart sides form a structure for attaching the motor. When glued to the base, they form two legs of an equilateral triangle.

3. Propulsion System: Cut the motor mount from a piece of cardboard 4cm x 12cm. The corrugations should run lengthwise, perpendicular to the folds. Extra glue is recommended because the motor tends to break loose from its mount in collisions. Glue the propeller onto the motor shaft and attach the motor mount to the aircart sides.

4. Assembling the Frame (Fig. 4): Glue the sides, with the motor in place, onto the triangular beams of the base.

5. Wheels: Pop rivets may be used for bearings in the wooden wheels. Pull the rivet to the center of its shaft, and push the rivet into the hole in the wheel. Both ends of the pop rivet shaft can be glued directly onto the bottom of the aircart.

6. Steering Assembly: A steerable front wheel is highly recommended. The steering arm can be built up in four layers. The arm itself, with the front wheel attached, is on the bottom. Two pivot disks are glued on top of the arm. This pivot disk fits up into a circular hole cut in the base of the aircart, and then a cap disk is glued on top to keep the whole thing in place.

7. Propeller Guard (Fig. 5): A propeller guard will help protect your fingers and improve the structural strength of the aircart. The guard is a piece of clothes-hanger wire bent in a circle with a radius of 7.5cm.

8. Wiring (Fig. 6): After the motor assembly has been safely glued in place, you can add a battery and a switch and then wire it up. Soldered electrical connections are recommended.

9. Testing
Average Speed: Calculating the average speed is easy. How "fast" is your aircart? How fast will it go in one meter? How fast in three seconds?

Changing Speed (Fig. 7 & 8): To study the changing speed of the aircart, several sequential time measurements must be made. You can use a lap-timer stopwatch (Radio Shack #63-5013, 9 lap timer $25) to time the aircart as it moves along the floor, or if a Microcomputer-Based Laboratory apparatus or the CCSR (see "Making Smarter Probes," page 5) is available, you can use use it to study the aircart moving a shorter distance on top of a lab bench.

To measure changing speed with a stopwatch, set up a course for the aircart by marking distance intervals along the floor. Use uneven distance intervals - short in the beginning and longer near the end. We tried a 15 ft. course, starting with a 1 ft. interval and ending with a 5 ft. interval. That approach gave reasonable timing accuracy.

If you have MBL capability, you can get velocity information about the aircart using a "smart pulley." We had the aircart move about half a meter along a table top.

The aircart and other HOP units are written in HTML format and are available at our web site. HOP units are appropriate as the basis for a complete course in physics or may be used individually in conjunction with more traditional approaches.

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