Design Your Own Experiment Lab Report Objective While making our water rocket, our group tried to achieve different criteria listed by Mr. Happer. With our rocket, we were trying to achieve a distance of at least 10 meters into the air straight up without tumbling, have a parachute deploy from the rocket successfully, and have an egg passenger go higher than 10 meters into the air and return safely on the parachute. Journal Entries 1/18/10 Today Mr. Happer told us to use the following physics vocabulary words and relate them to our experiment: Inertia: The rocket with be acted on by different forces (e.g. gravity and air resistance) therefore it will follow the law of inertia and either stand still or continue in a straight line unless acted on by the forces. Mass: The rocket s water (propulsion material) adds mass to the rocket. When it s expelled from the bottle by air pressure, this mass pushes back on the air inside the bottle to propels the rocket upwards. Weight: The weight of the rocket must be at a place (maybe the top) so it won t tumble around in the air (and so it will land right using the parachute). I understand that the center of gravity should be above the center of where the outside air pressure acts to make sure that when the rocket rotates it has an air force that will cause it to straighten up instead of tumbling. Force: We will need enough upward force propulsion to get the rocket up to 10 meters. We will also need to have the force of the parachute deploying counteract the force of gravity and slow the rocket down as it is falling to the ground. 1/20/10 Figure 1 Original design of deploying parachute
Today we thought of ways in which we could deploy the parachute from the rocket while safely bringing the egg astronaut down to safety. We needed it to match two requirements we made for the parachute. They were: having the parachute deploy at the peak height and having the parachute stay on the rocket during release as shown in figure 1. We thought of a method where we would have a long string attached to the cap holding the egg and parachute inside, and at the peak a person would pull the string and the cap would come off allowing the parachute to come out. 1/22/10 Figure 2 Prototype of parachute Today we thought of ways we could make a parachute that would slow down the rocket and its egg passenger in it s descent. We found that parachutes worked with the force air drag, so the more air drag, the slower the rocket would go down because it would be counteracting the force of gravity. This meant the bigger the parachute, the more air drag it would have and the slower it would fall down. Therefore, we needed to make a parachute that was light yet had a large surface area. One material we thought might work was a plastic garbage bag because it is very light. We made a parachute then connected it to a plastic weight that we thought was about the weight of an egg as shown in figure two. We tried this new parachute out however, even though it brought down the plastic weight slowly, it was very prone to rips which would mean we would have to make a lot of
parachutes. Now we are thinking of different materials we can use to make the parachute without it breaking. Possibly cloth. 1/26/10 Figure 3 Weight added to substitute for egg and used later for extra weight (adding inertia) for rocket Today Sherman and I merged with Luke. We tried our parachutes by throwing them off the second floor with weights attached. Sometimes the parachute lines seemed to get caught and did not deploy therefore it just dropped straight down and hit the ground hard. We needed a way to deploy the parachute so it will hit the ground lightly so as to not damage the egg. 1/28/10 Figure 4 Rocket used to act as weight for dropping with parachute
Today we thought more about the parachute and how it would bring down the bottle rocket. We brought in a more sturdy cloth parachute today that we thought would work well. We made a prototype rocket with the fins on. The fins were made of a sturdy type of Styrofoam and taped on using duct tape. We just wanted to have a bottle that we could drop to see if the parachute would work. We dropped the bottle from the third floor and thee parachute worked very well but we took the fins off the rocket because they were too high up on the rocket and would not keep the rocket straight when it flew because the center of air gravity would be high. 2/3/10 Figure 5 Final design of water rocket Figure 6 Final design of egg capsule
Today we built a new rocket because someone had taken our old rocket and cut it apart. This time, we made the fins lower so the air could push it back in flight if it started to turn and keep it going straight up into the air. We used the bottom of our broken rocket to make a small capsule for the egg and possibly the parachute. We launched this rocket and found that the parachute did not deploy when we put it inside the capsule. Therefore, we decided to just drape the parachute over the top of the rocket and see if it would deploy if it hopefully stalled at its peak. 2/5/10 Today we tested our rocket with the parachute draped over the rocket, hoping it would deploy at the peak of the rockets flight path. However, we discovered that the rocket did not go as high with the parachute draped on it possibly due to air drag. We found that as the rocket went up in the air, it went sideways. This could be due to the fact that it was windy or the fact that the launch mechanism sometimes tilted when the holding device was pulled from the clamps holding the bottle. To solve this problem, Mr. Happer told us to tape a straw on the side of the rocket vertically then put a rod into it and into the ground so it direct the rocket straight up at launch. This seemed to help, however, the rocket seemed to still get blown sideways. This could have been because when the rocket released all of its water (which is weight) it lost mass and Newton s 2 nd Law states force=mass*acceleration. Therefore, the lower the mass (the rocket after launch has a smaller mass) the more it might be accelerated sideways by the wind force. We decided to put more weight on the rocket because even though more inertia means it is harder to accelerate it, it also means the rocket will be harder to move in air and be acted on by forces like wind. Adding weight in water because the water makes the rocket go higher could compensate this trade off of adding more inertia (however we would still need to add other weight not in water). 2/9/10 Today, we tested out a new rocket that we made. This time, we put playdough at the top to act as a weight and increase the inertia so the rocket wouldn t be affected by wind. However, even though this worked and let the rocket go higher to about 10 meters, the egg did not survive the fall because the parachute failed to deploy. We had been hoping for the rocket to stall at its peak height so the reverse air drag would push open the parachute. However, the rocket did not stall but kept going in a nose-down arc. Also, the capsule holding the egg did not detach therefore, the rocket took a nosedive and landed on the capsule and the egg broke both times we used it. This could have been because of the increased weight of the rocket at the nose which made it tip over after reaching its peak height, not allowing air to fill the parachute and cause it to deploy. We needed to find a way to make the rocket stall
at peak height in order for the parachute to deploy. Mr. Happer recommended having the capsule holding the egg and parachute separate from the bottle so it would be easier to come of and let the parachute deploy. One way to do this could be attaching metal rods to the sides of the bottle going through straws so the capsule rests on the straws. The rocket would go up with the capsule attached because of the force holding the capsule down. However, when it started to fall down, the capsule s straws would slide off the rods and detach from the rocket allowing the egg to float down safely. This is shown in figure 6. Capsule holding parachute and egg Straw attached to capsule but not to rocket Rods attached to rocket but not to capsule Bottle rocket Figure 7 Possible design of new rocket
12.7 cm Straw to hold rod Parachute 50 cm in diameter 12.5 cm 9.5 cm Rubber stoppe r 2-prong launch stand Figure 8 Final design dimensions The rocket is a 2-liter sprite bottle. It is 10.15 cm in diameter and 31.1 cm long. The approximate weight of all pieces added together is Note: all 3 fins are the same size.
Free Body Diagrams Stationary Diagram Support force (in this case is the cork and the two-pronged stand holding the bottle up. Force of gravity pulling the bottle down. Figure 9 Free body diagram of a stationary rocket When the rocket is stationary, there are only two forces acting on it. They are, gravity and a support force. The bottle rocket is stationary; this means it has a net force of zero. A net force of zero means the two forces (support force and force of gravity) are equal; therefore, the two arrows representing each force are equal length. Moving at a Constant Velocity Force of thrust or propulsion from the water in the rocket pushing it up. Force of gravity pulling the rocket down. Force of air drag or air resistance pulling the rocket down. Figure 10 Free body diagram of rocket moving at a constant velocity
When the rocket is moving at a constant velocity, its speed and velocity are unchanging and the net force is 0. This means the forces (thrust, gravity, and air drag) acting on the rocket are at equilibrium and are balanced, therefore, the two arrows pointing down are equal in length to the force pushing the rocket up. Accelerating Force of thrust or propulsion from the water in the rocket pushing it up. Force of gravity pulling the rocket down. Force of air drag or air resistance pulling the rocket down. Figure 11 Free body diagram of a rocket accelerating When the rocket is accelerating, the velocity and speed are changing, therefore the net force is pointing upwards. This means the forces (thrust, gravity, and air drag) acting on the rocket are uneven as the force of thrust is greater than the opposite forces in order for it to get faster. Therefore, the arrow representing thrust is larger than both the gravity arrow and the air drag arrow combined because the force of thrust is larger to accelerate the rocket.
Reflection Jacob Dyer Throughout the project, as we developed our design, we used a lot of the physics concepts we learned from chapters 2-4 in the Conceptual Integrated Science Explorations book. It helped us to both further understand the science behind the bottle rocket and ways in which we could better fulfill the criteria for our rocket. The main problem for our rocket was the parachute. The rocket could consistently go to or above 10 meters without a parachute, however, when the parachute was added, it did not allow the rocket to go as high. One of the questions we had about the experiment was, why is it so hard to deploy the parachute? During the experiment, it was very hard to deploy the parachute because it did not meet requirements we set for it. We needed the rocket to go straight up into the air and then stall because we wanted to change the direction of the force acting on the capsule. To do this, we wanted the center of gravity high enough so the rocket didn t tumble and low enough so it would stall. Instead of having the air drag (air resistance) pointing down on the bottle, we wanted it pointing to the top of the capsule at the point the rocket reached its peak height for it to separate the capsule from the rocket. The force of the wind was another reason the parachute didn t deploy. The wind was always coming stronger from the top making it impossible for the parachute to deploy because the wind kept it stuck to the rocket. Therefore, the only way for the parachute to deploy in this design would be for the air drag force on the rocket to be opposite and come from the bottom of the rocket. Also, during launch, the capsule on the rocket never seemed to reach the 10-meter height needed to pass criteria off because the capsule would get blown off by the large force of propulsion/thrust during launch (as the capsule was not attached securely so it could come off easily later in the flight). The capsule was blown off because it was not rigid enough in the sideways direction. Another problem that occurred in our experiment was the wind blowing the rocket off course after it had emptied out the majority of its water at launch and had less weight (which is mostly water) and gravitational pull to the earth. In order for us to stop the rocket from getting blown sideways by the wind, we had to increase the inertia on the bottle because more inertia means it is harder for it to change direction. It also means having more mass, as inertia and mass are proportional and equivalent. Newton s 2 nd Law states the sum of forces=mass*acceleration. Therefore, the lower the mass (the rocket after launch has a smaller mass than it does before launch, filled with water) the less force it will have and the easier it will be to accelerate it in any given direction. We decided to put more weight on the rocket as shown in figure 3 because even though more inertia means it is harder to accelerate it, it also means the rocket will be harder to move in air and harder for it do be affected by forces like wind. Solutions to this problems included both adding weight in water (however there is a trade off, more water is more energy but more water is also more inertia making it harder to move) because that gave thrust to counteract the opposing forces and propel the rocket higher and adding a weight that would stay on the rocket throughout the flight. As the rocket is launched, it will continue to accelerate until the forces of gravity and air drag equal the upward
thrust on the bottle then it will go at a constant velocity (until there is no thrust) as shown and explained in the free body diagrams, figures 10 and 11. Newton s 1 st law states an object in motion will stay in motion unless acted on by another force. When the rocket is at rest, it is at equilibrium and the net force of gravity and the support force is zero, as shown and explained in figure 9. When we launch our rocket, it stayed in motion until it was acted on by other forces, which in this case were thrust, air drag, and gravity. This made our group think about ways to keep the rocket accelerating straight up for as long as possible as this would mean it would go the highest it could go. This made us make decisions such as putting more water into the bottle because that would give more thrust allowing the upward forces to counteract the downward forces for a longer time. Newton s 3 rd law states for every action, there is an equal and opposite reaction. This law explains how the rocket works because it shows what action causes the rocket to shoot up very high into the air. The system for the bottle rocket is: the air inside the bottle pushes down on the water, at the same time the water pushes up on the air, so then the air inside the bottle pushes up on the bottle. The force of thrust becomes greater than gravity so the net force is up, thus making the water rocket go up. This is also shown in figure 11 the free body diagram of the rocket accelerating as the arrow for thrust is greater than the arrows for both gravity and air drag combined. This helped to influence our decisions about the rocket because it made us try to pump as much air into the bottle as possible because we knew that this would make more air push down on the water, more water push up on the air, and then more air pushing up on the bottle creating more thrust and causing the bottle to go much higher. This experiment helped me to really nail in physics concepts because we were able to see the science behind the concept in real life instead of reading it from a book and we got to use these concepts we learned to help make our experiment fit the criteria (e.g. pumping as much air into the rocket as possible to make it go higher). Even though my group did not get the top criteria for the rocket and reach 10 meters and having a parachute deploy so as not to harm the egg passenger, overall I think this was a great, fun, practical learning experience that has deepened my understanding of physics as a whole and compels me to try and achieve the last criteria after Chinese New Year break.
Videos Jacob Dyer Figure 12 Successful deploying of parachute however the rocket did not go high enough Figure 13 Successful deployment of parachute however the rocket did not go high enough