Students design and make cars that work by having the motor conected to a wheel. The wheel may rest on the ground making it a direct-drive car. Alternatively, the wheel on the motor may rub against one of the car's wheels with friction between the two wheels causing the car to move.
Photocopy worksheet, "Designing a Direct-drive Car" and "Designing a Friction Drive Car". Download below.
Motor, AA battery, wheel (red, yellow, blue or green), and bushing – one of each per student
Aluminum foil, wire, rubber bands, paper fasteners, tape, cardstock (for making switches and connections)
Battery holders and switches previously made by students (Lessons 1 and 2), or newly made.
Friction-drive car materials: cars made in Lesson 4.
Direct-drive car materials: wheels (black), skewers, straws, cardboard rectangles and strips.
Two-gallon reclosable plastic bags for storing work-in-progress.
Brainstorming electric cars: Review what students have done so far:
They have gotten a motor to run from a battery;
They have learned how to control a motor using a switch; and
They have made a car that can run by gravity.
The next challenge is to make a car that runs from a motor. Ask:
What are some ways you could use the motor to make the car go?
Students may suggest using a propeller, attaching a wheel to the motor, using gears, using pulleys, etc. There are four ways we’ll be exploring in this class:
1. Direct-drive: A wheel is attached directly to the motor. By turning this wheel, the motor makes the car go, like someone pedaling a bicycle or tricycle.
2. Friction-drive: A wheel is attached directly to the motor and placed so it rubs against one of the wheels of the car. This turns the car wheel so it makes the car move.
3. Propeller-drive: A propeller is attached directly to the motor. The air blown by the propeller makes the car go, like a propeller-driven plane or boat.
4. Belt-drive: The motor turns a pulley, which is connected to another pulley by a rubber belt. The second pulley is what makes the car go. The belt is similar to the chain on a bicycle, and the two pulleys are similar to a bicycle’s sprockets, one on the pedals, and the other on the rear wheel. The differences between pulley-belt and sprocket-chain systems are that a belt is made of smooth rubber, and the pulleys have smooth surfaces.
Explain that we’ll begin by making direct-drive a d friction-drive cars. There will be time later to make either the propeller-drive or the belt-drive car, perhaps both.
How does a direct-drive car and a friction-drive car work? Explain that the large wheel will be the one that makes the car go. It will have to fit tightly on the motor, so that when the motor turns, the wheels turns with it. Show what happens when you try to put the wheel directly on the motor shaft. It is too loose. Demonstrate how the bushing makes a tight fit between the motor shaft and the wheel. Now the motor can transmit power directly to the wheel, which will make the car go. This video shows how to attach bushing and wheel to the motor. Then present the challenge:
Design a car that will be driven by the wheel that’s turned by the motor.
What parts will you need? (See this video). Distribute worksheets. Ask students to list the materials that they think they will need, and to draw and describe their preliminary ideas.
The linked table shows how to address a variety of issues that can come up in making direct-drive cars.Each issue may have more than one possible cause, so multiple causes are listed for some of the issues, as well as a fix for each one. Here is a Table of Issues, Causes, and Fixes. Do not share any of these ideas directly with students. Provide just enough information to prevent frustration. The best way to do this is to ask questions, such as,
"Did you troubleshoot all the electrical connections?"
"Does the drive wheel turn when the car is off the ground?"
"Does the drive wheel still turn when you set the car on the ground?"
"Is the drive wheel touching the ground?" and so forth.
These videos show some of the issues and how to address them.