I teach 3rd through 5th grade science lessons and one of my favorite new lessons has been teaching force, motion, energy, and the scientific method with 2 liter water bottle rockets. It is a somewhat involved lesson in which we try to determine how much water in the bottle will make it fly the highest. The rockets are a 2 liter bottle with a ring fin, a Nerf football nosecone, and a quick-release nozzle. I built a Gardena style launcher that attaches to a garden hose and a bicycle pump in which you can fill the bottle with a predetermined amount of water (measuring with graduations marked on the launcher guide posts). We pressurize the bottles with 60 psi of water each launch, but vary the amount of water in 0.2 liter increments.
After we have launched and timed our rockets with 0.2 through 1.8 liter volumes of water, we plot the time aloft on the Y axis and the volume of water on the X axis of a graph for easy and visual data analysis.
Here is a link to the water rocket and launcher instructions:
The forces acting on the rocket are the acceleration of the water through the nozzle towards the ground, gravity, and wind resistance/friction. The potential energy is the compressed air in the volume of the bottle not taken up by the water. The less water you have the more energy in compressed air you can store, and your rocket will be lighter at launch, but air does not have as much force to propel the rocket, as its mass is so much less. Force = mass X acceleration, so if what you are accelerating out the nozzle does not have much mass, the launch thrust force will be reduced. In other words, it’s a balancing act, and the fun part of this experiment is to find the perfect balance of water and compressed air.
Here is where the story gets interesting. I taught this lesson for the first time about a month ago in my first summer session, and I expected this nice dome shaped graph, where 0.2 and 1.8 liters had the shortest time in the air, and at some point in the middle there would be a peak. We launched 2 rockets at each volume, and indeed my expectations were correct, with the exception of the 0.6 liter launches. There seemed to be a dip there on both launches. I disregarded it and assumed that I had not put enough air pressure in the rockets for those launches. One student had measured a time that was greater than the other students, so I made that dot really big on the graph, and put my X on the graph where my intuition said it should be. In reality you can see the data points for the two launches clustered in two groups of three dots well below the point at which I placed my X. With that data analysis, we concluded that 0.6 liters is the amount to put into your rocket to make it fly the highest.
Fast forward to my second summer session last week, in which we did the same rocket experiment. In that class we came up with very similar data, and as I was plotting it on the graph for the class I was thinking in my head that I must have not put enough air pressure in the 0.6 liter launch when I remembered the data from the first session. This time I respected the data, and plotted it as it was recorded, and you can see very clearly the 0.6 liter dip. We even plotted the data points for both launches for 0.6 liters on this graph. This time we concluded that 0.8 liters is the amount to use to make the rocket go the highest.
Now the question is WHY? That just doesn’t make sense to me. I am going to run this experiment on my own with 0.1 liter graduations and very accurate measurements and see if more accurate measurements give me more insight. My initial thought is that there is a longer coast after thrust due to a higher initial velocity involved at around 0.4 liters or so, but I am going to have to research this.
The takeaway lesson here is what Paul Simon told us years ago, “A man hears what he wants to hear and disregards the rest.” There is no place for this in science, although for 3rd through 5th graders it really did make the lesson a lot easier to understand. This type of thing happens occasionally even in the higher levels of science, but my job as a science teacher is to teach kids to only trust the data, and to disregard our preconceptions of what the outcome should be. I am glad I caught this in the second session, and I will work this concept into my rocket lesson in the future. Science is awesome!