OverviewIn this challenge, students build a rubber-band-powered rover that can scramble across the room. Students will follow the engineering design process to design and build a rover out of cardboard, figure out how to use rubber bands to spin the wheels, and improve their design based on testing results.
- Read the challenge sheet and activity details to become familiar with the activity
- Gather the materials for each rover
- Build a sample rover
- Introduce the challenge (5 minutes)
- Tell kids some of the ways rovers will be used on the moon:
NASA plans to land astronauts on the moon by the year 2020. The astronauts will need moon cars—called rovers—to drive across the moon’s surface, carry supplies, help build their outpost, and explore the area. Today you’ll build and test a rubber band-powered rover.
- Show kids your sample rover and tell them:
This is a prototype of a rover, just like the one you are going to build. Prototypes are used all the time in engineering. They give you a basic design to build, test, and evaluate. Once you understand a design’s strengths and weaknesses, you can then find ways to improve it. Today, for example, as you test your rover prototype, you’ll find ways to make it work better. Improving a design based on testing is called the engineering design process.
- What do we have to do to make the rover move?
Turn the wheels to wind up the rubber band. Place the rover on the floor. Then let go. Note: Depending on the direction you wind the wheels, the rover can move either forward or backward.
- How can you make different kinds of wheels?
Kids can make different-sized wheels by cutting larger or smaller squares or make different-shaped wheels by trimming the squares. NOTE: Square wheels offer two advantages: they’re quick to make, and it’s easy to find the exact center by drawing diagonal lines. The center is where the lines cross.
- How do you think square wheels affect how the rover moves across the floor?
The points of the squares dig into soft surfaces, such as rugs, sand, or grass. This improves traction—the ability to grip a surface—and helps prevent the wheels from spinning out. Since the moon is covered in a thick layer of fine dust, good traction is essential, especially going up and down hills.
- Distribute the challenge sheet and instruct students to follow the directions in the "Build" section.
- Make the rover body - Fold the cardboard into thirds. Each part will be about 2 inches (5 cm) across. Fold along (not across) the corrugation (the tubes inside the piece of cardboard).
- Make the front wheels - On the two 5-inch (13-cm) cardboard squares, draw diagonal lines from corner to corner. Poke a small hole in the center (that's where the lines cross). On the body, poke one hole close to the end of each side for the axle. Make sure the holes are directly across from each other and are big enough for the pencil to spin freely.
- Attach the front wheels - Slide the pencil through the body's axle holes. Push a wheel into each end. Secure with tape.
- Make the rear - Tape the straw under the back end of the rover. Slip a candy onto each end. Bend and tape the axle to stop the candies from coming off.
- Attach the rubber band - Loop one end around the pencil. Cut small slits into the back end of the body. Slide the free end of the rubber bands into the slits.
- Wheels don't turn freely - Make sure they are firmly attached to the axles and are parallel to the sides. Also make sure the holes punched in the cardboard body are directly across from one another and are large enough to allow the pencil to turn easily.
- Won't travel in a straight line - Make sure the axles are straight and the front wheels are the same size. If one wheel is smaller, the rover will turn in that direction.
- Doesn't go far - Have kids wind up the wheels more. Also have them try using larger wheels. Bigger wheels have a larger perimeter (outer edge). As a result, one rotation of a large wheel will move the rover farther than one rotation of a small wheel.
- Wheels spin out - Wheels spin in place when a rubber band delivers too much power at once or when there’s not enough friction between the wheels and ground. To increase friction, have kids add weight over the drive wheels or add more wheels to each axle. To reduce how quickly a rubber band releases its power, kids can reduce tension by using a rubber-band chain or by cutting open a rubber band and using only a single strand of elastic.
- What kinds of Earth vehicles are similar to rovers?
Snowmobiles, tanks, dune buggies, and all-terrain vehicles are similar. They all have good traction, are very stable, have powerful engines, and don’t require a roadway.
- The challenge sheet gave you a rover prototype to get started with. How did starting with a prototype help you end up with a rover that worked really well?
With a prototype, kids can quickly see what’s working and what isn’t. They then know where to make improvements.
- How did friction affect your rover?
To be efficient, there needs to be minimal friction between the axle and the axle hole in the cardboard. To move, there needs to be lots of friction between the wheels and the ground.
- How did the rover use potential and kinetic energy?
Potential energy is energy that is stored. Kinetic energy is the energy of motion. Winding the front wheels increased the amount of potential energy stored by the rubber band. When the wheels spin, this potential energy is turned into kinetic energy, and the axle and wheels turn.
- How does the story about rover wheels on the back of the handout make you think about what it takes to design a wheel that can work on the moon?
Kids see that engineers face special design challenges when developing equipment to be used in space.
- Graph how increased potential energy affects distance traveled.
Kids can measure how far a rover travels as its rubber band is increasingly tightened. Have them turn the wheels 3, 6, 9, and 12 times and then measure the distance the rover travels each time. On a graph, have them plot the number of wheel rotations vs. the distance traveled. (Winding the wheels more increases the potential energy, which should increase the distance.)
- Determine the effect of friction.
Have kids wind up the wheels a set number of times and measure the distance their rover travels. Then have them minimize friction in the wheel-axle system. For example, they can line the axle holes with a material such as aluminum foil, then wind up the wheels the same number of times and retest their rovers. Use the following formula to calculate the percent increase in distance traveled:
Percent Increase = [(Distance modified rover traveled) - (Distance basic rover traveled) / Distance basic rover traveled] x 100
- Test the effect of wheel shape.
Starting with square wheels, have kids measure how far their rovers travel. Then have them snip off the corners of their wheels and test again. Make sure they wind up the wheels the same number of turns. How did the distance change? Did the wheels spin out? Test square, octagonal, and round wheels.
- Calculate the strength of your rubber band.
For high school students, have your class utilize the conservation of energy principle to determine the elastic constant of their rover systems. First, have students determine the mass of their rover and the time it took to drive across the room (velocity equals distance over time). Now students can use the conversion of kinetic energy from elastic energy to solve for elastic strength. This can be done using the equation ½ mx2 = ½ kx2