Project 3: Bottle Rockets
Spring 2009
Samantha Shropshire, Ray Barsa, Joseph Seymour
This activity is designed to help teach middle-school aged students some basic fluid mechanics. Using holes of different sizes in the bottle caps, students can easily see the difference in fluid velocities from the same Diet Coke and Mentos reaction inside of the bottles. The point of the activity, it should be noted, is not to teach the students about the reaction, but instead about the mechanics of the fluid flow.
Students should first be introduced to the mathematics of the nozzles. From conservation of mass, we can derive:
This means that, if we put the same amount of Diet Coke and Mentos into each bottle, the velocity of the fluid exiting each hole will be different, because the size of each hole is different. It also means that if the hole is smaller, the exit velocity is larger.
We also know, from Bernoulli's Equation:
Where P1 is the pressure inside of the bottles, ρ is the density of the soda, and z2 is the distance from the top of the soda inside of the bottle to the top of the bottle cap. While we can solve it, what's important is that conceptually, it tells us that the pressure inside of the bottle is higher than pressure outside of it.
We also know, from basic kinematics, that:
For students who have algebra, instructors could provide numbers for them to plug into the above equations, so that they can get a better feel for the relationship between velocity and nozzle areas.
After introducing students to the math, we can have them perform the activity. Students should be divided into groups of three or four students. They should be provided, first, three bottles of soda, a utility knife, and a ruler. They should first use the knives (they could also be provided awls or something else that is good for piercing holes, if the instructor is uncomfortable supervising a room of students with knives) to pierce holes of three different sizes on the bottle caps. They should measure and record the size of each hole.
Next, students will be provided with sheets of cardboard or poster board that are at least a few feet long. They should mark on the board every two inches, starting from the top of the soda bottles, so that they can prop behind the board when the reaction occurs to measure the stream's height.
At this point, the class should move outside. Preferably, the activity will take place against a wall, out of the way of bystanders. The activity is messy, so it's better to not annoy passers-by. Groups should set up their measuring boards against a wall, and then, one bottle at a time, put the same number of mentos into each bottle. They should then observe and record the height to which each stream of soda goes and the time it takes the stream to reach that height.
Once the students complete the activity, they should go back inside and calculate the velocity of the liquid exiting the bottles. They should use this to verify the equations are correct. They can eat the leftover Mentos.
If instructors want this to be a more engineering-focused activity, students could also be asked to design a method for inserting the mentos into the bottle simultaneously. They could also be required to design an object to hold the bottles in place while the reaction is going on, using items readily available, such as electrical wire or cardboard.
In the course of the activity, students should learn:
While not the purpose of the activity, the actual Diet Coke and Mentos reaction is an interesting side note. It is not primarily a chemical reaction. The surface of the Mentos is extremely rough, on a microscopic level. The holes in the coating of the Mentos are the correct size for nucleation to occur. The holes allow bubbles of carbon dioxide to form very rapidly, which causes the foaming jet. Rock salt is reportedly an effective substitute for Mentos in the reaction.
We decided that we wanted to teach students some basic fluid mechanics. During Moving 4th into Engineering and Meeting of the Minds, we learned that both young kids and adults actually know very little about fluid mechanics- both groups, before either performing or hearing about the activity, didn't always know that the velocity of a fluid leaving a nozzle is larger when the size of the nozzle is smaller.
We decided that we wanted to create an activity that could help people grasp this fundamental concept that was entertaining, easy to do, and easy for an instructor to run for a group of any size. This constrained us to an activity that didn't require a large amount of objects to be prepared beforehand.
It's surprisingly difficult to design an activity that is both a good example of rapid prototyping techniques and an activity that is engaging and educational for kids. Especially in this case, where we were asked to design an engineering-focused activity for middle school-aged students, it seemed to require each group to make a choice- either the group could prototype something, or they could design an activity that would be easy and engaging for kids to participate in.
In the course of the project, we built a launcher for the bottles out of PVC and wood, and drilled holes in nearly innumerable soda bottle caps. We also learned about appropriate selection of experiment materials, through a notable failure of club soda to provide an adequate reaction.