By Lindsey Dumond, III Form
III Form Physics: First Engineering Design Challenge
Units 1, 2, and 3 all focused on the same design challenge, which was to create an object with the highest average velocity over two meters. The object also had to be self-propelled and could not leave contact with the track at any time, or it would be disqualified. At the end of unit one, we chose several materials that we thought would be most useful in creating an object that could complete the challenge. We created a first design based on what we knew about physics at that point. In unit two, we built a model of our first design and tried to make it self-propelled. After facing challenges during this step we redesigned and rebuilt. As we redesigned, we were also learning more about physics and concepts that we could apply to our design and model to make it perform best. This process continued throughout the rest of unit two and unit three until we reached a design and model that worked and could complete the challenge.
We created our first design at the end of unit one, when we had already covered topics like acceleration and velocity. During our first design, we didn’t think much about the physics aspect other than that our object shouldn’t have a huge mass because it needed to propel itself, and that we needed to make it fairly aerodynamic. We knew that the larger the mass of the object, the more force would be needed to make it move, because of f=ma. We created an object with two axels made of straws that were connected by plastic balls and Popsicle sticks, with a wheel on each end of each axel (total of four wheels). When we were creating this first design we put off the idea of self-propulsion, so after we built the model of that design in unit two, we were stumped. Our first idea was to wrap rubber bands around the axels in such a fashion that when we released the rubber bands, they would contract and spin the axels in the process. We spent a good amount of time trying to figure out how this would work best. We tried a variety of different ways to wrap the rubber band and nothing was working. This was because the heavy wooden wheels made the mass of the object far too large and we would have needed a much larger force to move it, and the rubber band did not have enough potential energy in the ways we were wrapping it to propel such a big mass. The rubber band would need more tension and we would have needed to stretch it more to create more potential energy, but we couldn’t do this without bending our flimsy straw axels. Since we couldn’t figure out our way around the rubber band, we decided to try a new form of propulsion.
Going into our second design at the end of unit two, we were thinking of other ways to propel our object without a rubber band. The only other material that we had access to that could provide the amount of elastic potential energy that we needed to move our object was a balloon. We built a contraption made up of a plastic bowl and Popsicle sticks that we could put the balloon in to make it stay straight as it released air. Making the balloon release air straight would keep the object moving straight, which would give the shortest path from start to finish and therefor allow for the highest average velocity. Newton’s Third Law should have caused the object to move forward when the balloon released air because there needs to be an equal force moving in the opposite direction of the air, but the balloon did not have enough potential energy to move a mass that was still fairly large. We still needed our object to move somehow across the track so we essentially just attached it to the back of our existing model from our first design. We made the decision to keep the wheels because at the time we thought the balloon would provide enough additional potential and kinetic energy to move the object. In the end, the mass was still too high for a single balloon to propel, and when we tested this new model, it still did not move at all.
For our third and final design, we made the decision to start fresh, which would mean creating an entirely new design. From our first two designs and models, we could see that the extra mass from the four wheels was the thing that was holding us back the most. Going into the new design we wanted to make it simpler. We realized that previously we were thinking about it too hard, adding too many unnecessary additional parts that were just complicating the movement of the object. The new design we arrived at was about as simple as it could get: two balls and two rubber bands. We took two rubber bands and cut them, stringing each through a hole on the inside of each ball, and tying it back together on the inside. We strung the second rubber band through the first one before tying it inside the second ball, so it became one whole object. Our new design worked by displacing one ball to create tension in the rubber bands and increase their elastic potential energy and then releasing this ball. When the ball we had originally displaced made contact with the ball in front, we immediately released that one too, and the object accelerated forward. This design worked extremely well because of the transfer and conservation of energy between the two balls. Since energy must be conserved, all of the potential energy in the ball that was displaced was conserved and transferred into kinetic energy when the two balls collided. Conservation of momentum also played a big role, since no momentum was lost in the transfer from one ball to the next, but it was really the collision that did the trick. The lack of friction between the balls and the track was also a big factor in our success. Since we used spheres, the amount of surface area that was actually in contact with the track at any given time was very small, which greatly reduced friction. Our prototype testing showed us that the further we displaced the first ball, the more potential energy it had, and therefor the faster the object, as a whole, would move. At 20 cm displacement, the object took an average of .745 seconds to travel two meters. At 25 cm displacement, it took an average of .565 seconds to travel two meters. The difference is clear, and during our final run we chose to displace it as much as possible and got our fastest time yet of .31 seconds. There is not much I would change about this design, because it completes the challenge optimally and did what it was meant to do. I’m sure there are ways to reduce friction even more and give the rubber bands even more elastic potential energy, but in this case I do not think it is entirely necessary to do so, and I’m honestly not sure how we would go about doing this.