Build a Constant-Speed Vehicle
This project takes you through assembly of a model battery-powered vehicle from a kit. You can also build a constant speed car from scratch rather than from a kit, but some of the parts needed may be hard to find at local hobby stores or hardware stores. (The kit used here came from the Kelvin Company.)
Adapted from A. Gleue, Fun Physics Projects for Tomorrow's Rocket Scientists, McGraw-Hill Education, 2013
Things You Will Need
|Gear wheels (rear)
|Front wheels||2||Hobbyist kit|
|3-volts DC motor||1||Hobbyist kit|
|Pulleys||4||Hobbyist or hardware supplier|
|Rubber bands||4||Hobbyist or hardware supplier|
|AA battery holder (for 2 AA batteries)||1||Electronics supplier|
|Axle rods||2||Hobbyist or hardware supplier|
|Screw eyes||4||Hardware supplier|
|Wooden base||1||Hardware supplier|
- Pencil or pen
- Pliers or long-nose pliers
- Hot glue gun with hot glue sticks
- Wire strippers
- Soldering iron (optional)
- Drill with small drill bits (optional)
- Small nail and hammer (optional)
BE CAREFUL! Hot glue guns and soldering equipment can get very hot and cause serious burns. The electric drill can be dangerous, too. Always use caution when using any tool that can burn or cut you. Adults should supervise children's use of tools.
(The following steps describe one approach to assembling the Kelvin CV model racer shown in Fig. 1.)
1. Spread the components on a clean, flat surface and put similar items together.
2. Use the eyehooks to attach the axles to the car's body. Using the ruler, first make four marks on the weed base with a pencil. Measure 1/2 inch from the ends along both lengths, and 1/4 inch from the ends along both sides of the width of the rectangular wood base (Fig. 2).
3. Using these four measurements, draw four lines, with darker dots at the intersections. These measurements don't have to be exactly like what's shown in Fig. 2, but be sure to keep your lines straight.
4. Drill four small, shallow pilot holes for the eyehooks at the four dots you drew in step 2.
5. Screw the four eyehooks into the wood base. Rather than using a drill, you could also use a small nail and a hammer to create small pilot holes. You may need pliers to screw in the eyehooks securely. After the eyehooks are screwed in and aligned, make sure that your two axle rods fit into the eyes of the eyehook. (Look ahead at Fig. 4 for a preview of the finished wheelbase.)
6. Once you have the axles lined up, remove them and insert one small front wheel on one of the axles and one of the larger wheels on the other axle. The small wheel is for the front of the car, and the larger wheel is for the back of the car. The wheels should push onto the axles, although you may need to gently tap them in with a hammer.
7. Taking one of the plastic pulleys that came with the kit, insert the second-largest pulley onto the back axle so it is pushed close to the rear wheel; see Fig. 3.
8. Push the axles through the eyehooks, and place the second front wheel onto the front axle and the second rear wheel onto the rear axle (Fig. 4).
9. Once you finish the wheel assembly, push the car around to make sure it rolls in a straight line and with minor friction. Now you’re ready to work on the motor assembly.
10. Find the battery holder, switch, and motor. By making a complete circuit, these three devices make up the electrical and power-generation parts of your car.
11. There are four wires: two coming from the battery pack and two coming from the switch.Before you connect any of these wires, strip more from the ends using a wire-stripper so you have about ½ inch of stripped wire from each end.
12. Put two fresh AA batteries into the battery holder, and make sure the batteries are aligned correctly: positive (+) to + and minus (–) to –.
13. Take the red wire from the battery holder and twist it with one of the wires from the switch (see Fig. 5). One option is to use the solder and a solder gun to make your connections more permanent and durable. If you twist the wires together tightly, soldering may not be necessary.
14. Notice that the motor has two connections (A). Take the other wire from the switch and run it through one of these connections (B). Make sure to run the wire through the connection tightly and then twist the wire around the connection to secure it.
15.Now you’re ready to make the final electrical connection. Thread the other wire from the battery pack through the other motor connection (C). Again, tightly twist it so you have an electrically sound connection (D).
16. Turn on the switch and see the motor’s axle rotate. You have created a pathway for the electrical energy to run from the batteries through the switch to the motor. If the motor doesn’t work, first check that your connections are making good electrical contact, and then make sure your batteries are properly and securely placed in the battery pack.
17. Put the small pulley on the motor’s axle shaft (see Fig.6).
18. You are now ready to add the rubber band to the pulleys on the motor shaft and the rear axle. Put the battery pack, switch, and motor on top of the car’s wood body toward the middle and front of the car (see Fig. 7).
19. Put one of the rubber bands around the motor shaft pulley and then around the rear wheel and around the pulley on the rear axle. There should be some tension or tightness in the rubber band, as in Fig. 8, but not excessive tightness. If the rubber band is too loose, it will fall off the pulleys too easily. If the rubber band is too tight, there will be excessive pull against the motor.
20. Once you have the battery pack and motor in position, hot glue the motor, battery pack, and switch to the wood top of the car.
21. Retest your car’s switch and electrical connections. Make sure pushing the switch activates the motor and spins the motor shaft, turning the rubber band and the back axle. You may need to make some minor adjustments.
22. Put the car on a smooth floor and turn on the switch. The car should roll forward in a straight line. You may want to test the car on different surfaces. You might find your car moves better on carpet than on a smooth floor. If your car is not moving in a straight line, check the straightness of your axles; you might need to make minor adjustments to the eyehooks. Make sure the wheels have adequate spacing and are not rubbing up against the side of the car. Fig. 9shows two views of your new car.
Constant speed, although the most basic and straightforward type of motion, is an important concept. Many types of objects on Earth and throughout the universe have unchanging motion, that is, the ability to move with constant speed. You can graph time and distance data for this type of motion. When you do this,you get a diagonal line. The slope of this trendline represents the speed of the object.
You can also use the constant-speed equation to make predictions: just take out your camera and video something or someone moving. Doing a quick analysis of the video tells you how fast the object or person is moving. Can you think of something around you that might be moving at a constant speed? Maybe you see a bug or ant moving along the sidewalk, or a friend running, biking, or skateboarding along a smooth level surface, a car driving along the road, or a plane flying across the sky.
Pull out your smartphone or camera and take a quick video, and then analyze the data with a spreadsheet such as Excel or with the Tracker software. Graph the time on the x axis and the distance moved along they axis. Do you get a sloped, diagonal line when you graph it on paper, with Excel, and with the Tracker software? If so, the object is moving at a constant speed, and you can determine its speed by calculating its slope and come up with its equation. What about the motion of an object that changes speed? In physics, this is called acceleration.