Rocket Balloon Lab

Research Question

“Determine the work done and the power of a balloon rocket on the assumption that the balloon rocket exerts an average force of 0.5N.”

Introduction

In the world of physics, the definition of energy is “the ability to do work”. If one has energy, it does not mean that they are happy and energetic, but it actually means, that person is able to do work. Work is “the action of a force to cause displacement of an object”. It can be found by the formula… (nmsea [Forms of energy], n.d.).

Work (J) = Force (N) x Distance (m)

The unit for work is “joules”, the unit for force is “newtons” and for distance is “meters”. So, the amount of force applied multiplied by the distance it travels is equal to the force. This means that if you apply 100 newtons of force, but the object does not move at all, you are not doing any work. If you apply 5 newtons of force and it moves 100 meters, the amount of work you are doing is equivalent to 500 joules. Now, if energy is the ability to do work, what is “power”? Well, power is the rate of doing work or using energy”. When we hear the word “rate”, we associate it to “time”, so the amount of power you have is the work done divided by the time it took to do the work, which can be represented by the formula… (Wikipedia, [work], 2000).

Power = Work Done (J) / Time (s)

When we want to measure power, we use the letter “W”, in short for “Watts”. So, if we did 500 joules of work, but it took 10 seconds, than we used 50 Watts of power. (Wikipedia [power], 2011, December 3.).

This is a free diagram body of the forces acting on the rocket balloon. The force of gravity is pulling the rocket down, but the normal force exerted on the rocket ballon by the string opposed the force of gravity which keeps it from falling down. When we blow up the ballon and let go, it will move to the left (assuming that is the way it is facing) due to the applied force of the air rushing out of the opening, known as thrust, but there will still be forces acting against it such as friction and air resistance. Because the applied force is greater than the forces acting against it, it will accelerate and move to the left. (Purchon [physics at gondar design science, n.d.).

This is the reaction force pair diagram for the motion of the balloon rocket. As the air rushes out of the opening towards the right, the balloon will move to the left, because it will move in the opposite direction. What causes the air to rush out and move the ballon is the elastic potential energy being converted to kinetic energy. When we blow up the balloon, it stores potential elastic energy. The bloated balloon will want to return to it’s original state, so when we let the opening go, all of the air will rush out. Most of the energy will be converted to kinetic energy which will result in the balloon moving to the opposite direction as the air rushing out. Some of the energy will be converted into other types of energy like sound and thermal energy. These types of energy take away from the kinetic energy, but do not actually make the balloon move, which will result in less efficiency.

Video

http://www.youtube.com/enhance?feature=wenh&v=UUotGxyFMDA (Better video of actual trails)

Reliability

Although the way to alter your method was very limited, the method that I chose was very reliable. I was able to accurately measure the distance from one end of the string to the other, and I was able to measure how much to inflate the balloon. Unfortunately, my balloon popped about 3 times, and on the last few trials, I had to use a different type of balloon. This meant that because the size and elasticity of the balloon was a little different from my first few, the amount of air in the balloon might have been different, even if I did measure the length to exactly 35 cm. This would have affected the distance the balloon rocket would have gone. Also, I measured the time it took the balloon from the starting point to the ending point using a timer, but because the time it actually took was so short, even a tenth of a second in difference would have had a big impact on my results. In order to try to reduce the amount of outliers in my data, I tried to round the time it took to the nearest 10th. Because of this, all of my trials took approximately 0.3 seconds.  Fortunately, the straw and the string we used were consistent and so the amount of friction and thrust stayed the same.

Validity

Because the method of measuring the time it took was not very accurate, the amount of work and power may have been affected a little. Still, I made when I measured the time it took for all 3 trials and rounded to the nearest 10th of a second, all of them became 0.3 which tells me that this number is fairly close to the real time. This means that my data is still reliable to a certain extent. Also, the different balloon I used after all of the old ones popped, may have had more or less air than the original balloon which would have affected the amount of thrust (because the amount of air pushed out of the balloon would have been different). This would have affected the distance the rocket balloon would have moved. Luckily, looking at our data, there were no outliers that was too big, so it did not have much effect on our data. 3 out of the 2 trials ended up with distances between 6 and 7 meters, and the mean of the distances was also between 6 and 7 meters which tell me that the impact of the use of different balloons on our data was not too major.

Conclusion

The efficiency of a machine can be found by the equation…

work (output) / work (input) x 100

If the work output is equal to the work output, the machine has an efficiency of 100%, but theoretically, this is impossible because energy is always being transformed or transferred to something else. The less energy it loses from the target type of energy it wants to produce, the more efficient the machine is. For example, in the rocket balloon experiment, the elastic potential energy from the balloon was transformed into kinetic energy, but also into thermal and sound energy. The friction from the air rushing out and the surface of the rubber of the balloon creates heat (so the energy was converted into thermal energy). The vibration of the opening of the balloon when the air rushed out created sound (so the energy was converted into sound energy). All of these conversion of the initial energy into different types of energy other than kinetic, lowers the efficiency of the machine. Considering the fact that there are several other forces like friction and air resistance acting against the balloon rocket, it is important to try to convert as much of the initial energy into kinetic energy, as kinetic energy is the only type of energy in this experiment which makes the balloon move. In order to do this, we could have… (Forms of energy, n.d.).

1. Made the balloon “slicker” somehow by maybe covering the inside with oil, and the outside with tape. By covering the inside of the balloon with oil, we can reduce the amount of friction between the air rushing out and the surface of the balloon which means less energy will be converted into thermal energy. This means that less energy is lost, and more is converted into kinetic energy. By covering the outside of the ballon with tape (smooth tape with no holes and rips), we can reduce the amount of friction between the air and the balloon itself when it is in motion, which will in turn reduce the drag (air resistance). By doing this, there is less force acting against the balloon rocket, so it will cover more distance.

2. Connect the straw to the opening of the balloon (instead of taping the balloon to the straw). Because the straw is longer, it can direct the air to one direction more accurately and straighter. If it was just the opening of the balloon, some of the air will escape outwards and sideways which will reduce the amount of thrust in the right direction. By connecting the straw to the opening of the balloon, we are basically doing the same thing as to adding a longer barrel to a sniper. The longer the barrel, the straighter the bullet will fly, and the more accurate it is. The same concept is applied here. If more of the air is channeled to one direction, the amount of thrust will increase, and less force will be lost. Of course, because there is string going through the straw, we cannot put the balloon on that straw, so it will be better if we connect another straw on top of the straw with the string on it, and than tape the balloon to the straw on top. Also, by putting the straw on the opening of the balloon, the straw will not vibrate like it does when the air rushed out of the opening of the balloon itself, so less energy will be converted to sound energy. By reducing the vibrations, we are also reducing the amount of energy being transferred to thermal energy, so the machine will be more efficient overall.

Extended Conclusion

Throughout the assignment, we were to assume that the average thrust force of the balloon was 0.5 N, but this is not true. The thrust force is not constant throughout the run. There is a quick burst of thrust in the beginning, and then the momentum from the burst is what carries the balloon the rest of the way until friction wins. This means that there is not constant velocity. 0.5 N was what Mr. Taylor got from his lab, and we do not know if the amount of air he blew in the balloons for each trial was constant. He might have blew more air in the balloon for one trial, which means that there was more thrust force, which would then affect the data. By using the data that I collected, I was able to find out the actual thrust force using the mean velocity that I got from my data, and not the data that the teacher gave us. The real average thrust force is 0.17 N. I did not know the force in the beginning, but I did know the distance and the time. The average distance traveled was 6.4 meters, and it took the rocket balloon about 0.3 seconds to cover this distance. By dividing the distance and the time…

6.4 / 0.3 = 21.3 m/s (distance / time = velocity)

I was able to find out the mean velocity which was 21.3 m/s. Than I divided the mean velocity by the change in time time and got the acceleration. I did 21.3 divided by 0.3 which became 71 m/s/s.

21.3 / 0.3 = 71 m/s/s (mean velocity / change in time = acceleration

Finally, I multiplied the acceleration with the mass of the object which was 0.00245 g. The final average Force became 0.17 N.

21.3 x 0.00245 = 0.17N (acceleration x mass = force)

The thrust force that the teacher gave us and the thrust force that we found out was the average thrust force throughout the run. Unfortunately, the thrust force nor the velocity is constant, so the average thrust force would not  be very accurate or close to the real thrust force.

Self Evaluation

For Criterion B, I think that I got a 5-6 because I used sufficient scientific language and I communicated it effectively. I used in text citations, and had a list of references at the bottom (below the self assessment).

For Criterion C, I think that I got 5-6 because I used scientific ideas that we learned in class and constructed scientific explanations. I used the formulas correctly even in unfamiliar situations when solving the last problem.

I worked safely and required no guidance throughout the lab. I worked well with others in my group and had responsibility.

 

Citations