Yes, Bottle Cars! Not too long ago I was asked some questions about this amazing STEM challenge and I started searching for a blog post to recommend. Well, oh my goodness, I could not find where I have ever written very much about Bottle Cars!
So, let’s do it! I hope I can convince you to try this challenge.
Some expectations for you:
- You are going to need some empty water bottles- 16-ounce size works great.
- Expect to cut some holes in the bottles.
- Start thinking about the axle.
- The wheels, oh my.
- It’s all about that balloon (and propelling that car)
- You might need a track!
- Expect there to be some racing. #ofcourse
The Holes in the Bottle
This turned out to be an easy fix- after a learning curve for me. I added some tips for you below the photo. But first, here are the basics: The car is made of an empty water bottle. A large straw is placed through the bottle and students blow up the balloon and release it. The air in the balloon propels the car. Students must also design the axles and wheels for the car so that it will roll and not just scoot across the floor.
TIPS ABOUT MAKING HOLES: I always try to plan ahead for events like this. I know the bottles were going to need multiple holes. I also know that students would stab the bottles with scissors to make those holes.
- So, tip #1 is to let students know from the beginning that this is not allowed. My classes were told to mark the spots for holes with a sharpie and bring the bottle to my work table. I made the holes.
- Tip #2: Make sure the holes line up before you make the holes. If a car has two axles those axles must be parallel for the car to roll.
- Tip #3: I start a hole with the point of a compass and then make the hole bigger with a dowel stick or a Philips head screwdriver. A pencil will also work.
This is, by far, the most perplexing part of a car challenge. The axle has to turn to make the car move. The wheels are attached to the axle. The wheels do not roll independently of the axle. Think about your car. Why do the wheels turn? Because the axle is turning. Students will try to make a stationary axle and have the wheels rolling. Tips about attaching below.
TIPS ABOUT AXLES: Look at the above photo. It appears that the team has attached the straw axle to the bottle. But, look closely. Can you see the dowel stick peeking out of the straw? That dowel stick turns and the straw allows attachment of the device to the car.
- Tip #1: Thread a stick through a straw, attach the straw to the car body, and then attach wheels to the stick. This almost always works well.
- Tip #2: The best dowel stick to use is actually a shish-ka-bob skewer.
- Tip #3: If you use the skewer you should snip off the points first. I use a wire cutter.
The Wheels, Oh My!
Guess what? Most students cannot cut out a circle. They just cannot follow those lines in a circle and especially if they are using cardboard.
However, car wheels must be round. Tips below.
TIPS ABOUT WHEELS: Look at the photo. Those wheels look really good! They are almost round and I can say they will roll fine.
- Tip #1: Show kids how to trace a round object so the wheels are the same size. I use all sizes of cups for tracing.
- Tip #2: I show students how to hold the cardboard and turn the cardboard in order to cut out a circle.
- Tip #3: For my youngest students I cut out the wheels for them. I have a bag of about 50 cardboard wheels and I just give every group 4 and let them know I need them back! This makes the challenge more about creating a car than their ability to cut.
The balloon is the car’s motor. Before we begin this challenge I blow up a balloon and have students predict in which direction it will fly when I release it. Most of them will say the opposite of what actually happens. They need to know that the nozzle end of the balloon is the back end. The bulb end flies away first.
If they don’t get this, they will place their balloons on the track backward.
TIPS ABOUT BALLOONS: Some of these tips are things I learned the hard way.
- You need extra balloons. They will pop or they will need to be replaced.
- Each student in the group needs a balloon. They cannot share, obviously.
- Watch the balloons. You will notice when one needs to be replaced. Warning: this is gross. After blowing up a balloon multiple times you will see a collection of spit inside. Throw it away and start a fresh one. Trust me, you do not want one of these to pop.
This happened accidentally! With the first class to try these cars we just sat them on the floor to let them go. But, they do not always travel in a straight line and it is hard to race them.
So, I built tracks.
TIP ABOUT TRACKS: I used foam board purchased at Dollar Tree, which, by the way, is where you should always buy it.
- Tip #1: I cut the foam board in half and then cut off 4 inches on each side. The 4-inch piece was cut in half. I duct-taped two lengths for the track runway and duct-taped the smaller pieces as side rails. This kept the cars moving in a straight line.
- Tip #2: Make two tracks so you can have races. #mostfunever
These are really short but will show you how Balloon Cars work!
This challenge is challenging! Every team will not be able to design a car that will work. Some will work fine for one race and then fizzle in the second race. Some will only travel in circles. Some will always race down the track and win every race. I only use this challenge with 5th graders because it is difficult, but they LOVE IT!