99 Orange Rocket Balloons
16/12/2011 § 1 Comment
Research Question & Overview
The research question for this experiment is “Determine the work done and the power of a balloon rocket.” We have assumed that the rocket balloons in this experiment generally exert an average thrust force of 0.5N but we know that that is actually not the real average thrust force.
The units we used for this experiment were work, energy and power. Work is the action of a force to cause displacement of an object and is represented by J, joules. It can be found by multiplying force (N) by the distance traveled, or displacement (m). The equation to find work is therefore Work (J) = force (N) x displacement (m).
Energy is the ability to do work and is almost identical to work. Different types of energy include kinetic, mechanical, elastic, potential, thermal, sound, and others. Both energy and work are found the same way (with the same equation) and energy is represented also by J, joules. The equation for energy is therefore Energy (J) = force (N) x displacement (m). In context, however, work is the actual displacement of an object and energy is the object’s ability (actual and potential) to be able to move and “do work”.
Power is the rate of doing work or using energy. Power is the relationship of work (energy) and time. It is basically how fast or how slow an object can do work. Power is represented in W, watts, where 1 watt is equal to 1 joule in 1 second. Once the value of work/energy (J) is known, simply divide the time (seconds) to find the power. The equation to find power is therefore Power = work done or energy used (J) ÷ time (s).
At the beginning of the run, the forces that were acting on the balloon rocket were thrust, air resistance, normal force and gravity. While the normal force and gravity cancel out, friction and air are acting against the direction of the balloon. At the initial release of the rocket, the balloon exerts a lot of energy and releases lots of air. Because of this, the thrust force is strong; stronger than friction from the string and air resistance combined.
Towards the end of the run, the amount of friction and air resistance stays about the same. Air resistance could have changed because the shape of the balloon has changed and the surface area doesn’t hit as much air. By now, though, the rocket has lost most of its air and doesn’t have as much thrust as it did in the beginning of the run. This is seen in the arrow that points right (the direction of the balloon rocket). It is smaller than it was in the previous diagram.
In the reaction force pair, the elastic of the balloon presses against the air as it runs across the string. On the snapshot of the diagram drawn, as the rocket heads left, the elastic also moves that direction. The line demonstrating the air’s direction (pointing right) shows that air pushes against the elastic surface of the balloon.
The first law of thermodynamics is the law of conservation of energy. This means that energy is neither created nor destroyed. It can be transferred from one object to another or transformed from one form to another (Taylor, 2011). When the balloon is blown up with the air inside it, it hold potential elastic energy because the elastic of the balloon will tend to lose all tension to try to go back to its original, floppy shape. Also when the balloon is blown up (not many people may notice or remember this), there is also thermal energy transferred into the balloon which could be felt because the temperature of the balloon’s rubber went from its typical cold temperature to a warmer temperature. This thermal energy probably comes from the air that is blown into the balloon and is transferred into the balloon’s rubber material. When the balloon is released along the string, most of that potential energy is transformed into kinetic energy and this is seen in the thrust that pushes the balloon one way. The rest of the energy is transformed into thermal energy, elastic energy and sound that isn’t used in the work done by the rocket.
Method & Video
My group (which consisted of me, Kyu Jin and Alisa) created a method that would be the most efficient in that it gathered multiple units and types of data as quickly as possible. Below is the video of our method with a few notes of details we paid attention to, along with who in our group did what job.
The method was very simple, structured and efficient. We all knew our jobs (stated in the video at around 0:33 – 0:40), which made things easier. Basically, after we tied up a long length of rope on a railing and taped up the other end on a column, I blew up the balloon (Alisa would help me determine if each balloon was approximately the same size as the previous balloons) and released it. We didn’t bother un-taping the balloon from it’s spot on the straw and the string as that would have taken a lot of time. Instead, I blew the balloon from where it was on the string. Alisa used her iPhone to time each run. We took about 6 to 8 runs and out of that, chose the clearest ones. Kyu Jin helped measuring (also seen in the video) by using his fingers as a marker to indicate where to put the meter stick. We measured where the straw’s end started and where it finished (refer to video). Processing the data collected by this method simply consisted of letting Excel do all the math and using iMovie to process the videos from the iPhone to find out how many seconds each run took.
There was no independent variable in this experiment, nothing that we could manipulate to see its effect on a dependent variable. The variables that we did measure were work and power. (To find these, we found measurements for distance and time.) There were multiple controlled variables in the experiment. These were the slope of the string that the balloon rocket ran along, the amount of air that is in the balloon, and the tension of the string. An uncontrolled variable in the experiment is how much air is released as soon as the balloon is let go.
The above stated controlled variables were controlled while setting up the experiment and while doing the experiment. The slope of the string, for example, was manipulated by having someone adjust the string according to what the other group members roughly estimated was exactly 90˚ to the column. The amount of air that was in the balloon was controlled by me and Alisa mostly, where I’d blow the balloon but we’d both estimate, based on the size of the balloon, if there was the same amount of air in the balloon as in previous runs. Finally, the tension of the string was easily controlled simply by pulling the string tight and not letting it flop about. By making sure the string’s tension was very strong, we could be sure that the rocket balloon would run across the string smoothly without having to struggle because of a string that was loose and that wasn’t tense enough.
Data Collection and Processing
The results gathered from this experiment generally tried to stay within a certain range of measurements. These measurements are, for distance (in meters), about 6.70 meters to 7 – 8 meters (precisely 7.75 meters). This range is about one meter (1.02 meters, to be exact), which is actually a long distance because one meter is rather far. The measurements for time (in seconds) range from 1.8 seconds to 2.5 seconds, which isn’t even a full second. The relationship that we see in data is that, generally, the longer the rocket balloon travels (even a few extra deciseconds), the further it is displaced from its starting point on the line. The data does not totally support this, however, because if we arrange the values for time in ascending order (see table to the right), we can see that the time and distance don’t gradually grow together. Balloons 1 and 3 do ascend in terms of time, but descend in terms of distance traveled. The same situation occurs with balloons 2 and 4. The time increases by 0.1 seconds but the distance does not increase. Instead, it decreases by about 0.065 meters. In general, though, the most basic pattern is that the time travelled and the distance travelled are interlinked together in each rocket balloon run. This is because, logically, the longer amount of time something moves at whatever speed, the more distance it covers.
The error for the measurements for distance is 0.05 meters, which is also 5 centimeters. I gave the error bar for distance two decimals because we had Kyu Jin stand next to where the balloon would end and each time, he’d make sure that we’d measure to almost exactly where the balloon ended. The method we used was just precise like that. The error bar I used for the measurements of time was 0.2 seconds because I think it’s fitting enough. In iMovie, I was able to stretch each clip out and see each ½ second but iMovie also enables me to see things in 0.1 seconds so whenever I stop somewhere within the clip, it’s usually quite accurate.
I processed the data we already collected in order to find the work and power of each rocket balloon. The table below (‘Calculated work and power for balloons 1 – 5’) depict these calculations. The calculations used were the ones previously given in the Background Information section. We can see that in general, the amount of work (joules) that the balloons are doing ranges from within 3 to 4 joules. The calculated power for the balloon rockets in general ranged in between 1.50 watts to about 1.90 watts (1.87 watts, to be exact, see Balloon 1). Of all the balloons, this means that balloon 1 used the most power during it’s run. This makes sense because balloon 1, although it had one of the shorter displacement measurements, took the least time out of the five balloons. This means that it took more power for the balloon to get from the start to the end of its run in a shorter period of time while the other balloons had a little bit more time to run the length of their displacement. The error for the measurements of work and power were each 0.5 (joules and watts, respectively). This gives the data some breathing space just in case the measurements are a little off but it doesn’t try to be too exact either because our measurements are very raw and probably not exact as they could be with proper materials and more time.
Above is the collection of averages of all five rocket balloon runs of distance, time, work and power. According to the table, the average distance traveled was 7.11 meters but we have to remember to look back at the actual raw data. If we look at balloon 5, we’ll see that it’s a bit of an outlier compared to the other balloons at 7.75 meters, whereas the closest measurement next to balloon 5 is balloon 2, which is only at 7.38 meters. Also, balloon 3 is also a bit of an outlier as it is equally as separated from the middle three balloons as balloon 5 is. The time, work, and power averages are as stated, 2.22 seconds, 3.55 joules, and 1.61 watts.
One of the rocket balloon runs that we were about to film failed terribly because as soon as I released the balloon, the rubber stuck onto the concrete of the column and just stuck there for about half a second before actually running along the string. This reminded us that we had to make sure that the rocket was free of all disturbances before letting it go. Additionally, the length of the string limited the experiment a bit because we realised if the amount of air in the balloon passed a certain point, the rocket would just hit the end of the string (which happened to be a rail), bounce back along the string, and be a useless run. Finally, the string would move a lot along with the rocket.
Reliability and Validity of the Data
The data probably could have been more reliable than it actually seems. In fact, although our method was very consistent (if you refer back to the video, our way of measuring each balloon’s run was methodical and consistent; each member did the exact same thing each time which adds to the reliability of the method, therefore the reliability of the data gathered through that method). The raw measurements are reliable and quite valid but afterwards, the calculated work and power may not be as valid and reliable. The factor that most impacts the consistency and accuracy of the collected data is the amount of air that was used during each run. Since we couldn’t accurately measure an exactly amount of air for each time we blew the balloon up, the amount inside the balloon was definitely different for each separate run. Since the amount of air is the factor that affects most how far the balloon will travel and how long it will travel for, this is the primary reason why all the gathered data (distance, time, work, and power), although we tried to measure everything as precisely as possible, isn’t wholly and as reliable and valid as it could be.
Efficiency is a percentage, or a ratio to show the useful work out of the total amount of work done (Taylor, 2011). The equation to find efficiency is therefore: Useful work out (J) ÷ total work done (J) x 100%. In this case, the balloons are not being efficient because of where the energy is being transferred and what the original potential energy is being transformed into. The work done by these balloons is their displacement on the string so efficiency would mean a lot of energy, or as much energy as possible, put into the movement of the balloon. We know that good amount of the balloon’s original potential elastic energy is not transferred into the kinetic energy (the movement) of the rocket. Most of it goes to sound, elastic energy and thermal energy.
We also know that the rocket faces a few barriers on its run, more so at the end than in the beginning. This cuts down on the energy because it takes more energy to pass these barriers. During the entire run, the rocket has to deal with friction from the string that we used to hang the straw from. The friction is greater at the end of the run when the thrust force of the balloon isn’t enough to overpower it. At the beginning of the run, however, there was a lot of initial thrust that propelled the balloon to move forward with lots of energy. That energy eventually dies down, which causes the balloon to also slow down because not enough thrust can move it forward. At the same time, there is also air resistance because of the size of the balloon. It’s elastic surface area, as shown in the reaction-force pair in the Background Information section, is what hits the air as the rocket runs along the string. Newton’s 3rd law of physics is that “Every action has an equal and opposite reaction” and this law applies to this balloon experiment (Stern, 2004). This pushes against the forward movement of the balloon and if there was not as much air resistance, the rocket would have traveled further. All these factors add up to precipitate a less than 100% efficiency.
There are multiple ways to increase the efficiency of this rocket machine. The objectives are to obviously tackle the factors that take away from the efficiency of the rocket, like the ones previously explained.
- One method to increase the rocket machine’s efficiency and decrease the areas where energy is lost to different transformations is to reduce the air resistance that pushes against the balloon’s elastic surface area. A simple way to reduce the amount of air resistance on the balloon is the decrease the balloon’s surface area. There are multiple examples of how a smaller surface area makes movement faster. Planes and jets can cut through the sky and not find too much trouble with the colossal amount of air resistance they face at the speeds they travel because planes are quite thin. Similarly, skis and snowboards, when running down slopes, are quick because they are flat and the only area that is hitting air resistance is the front edge of each ski or the front edge of the snowboard. Just as well, if a swimmer makes sure that their bodies are straight in the water and as straight as line as possible, they will surely cut through the water a lot quicker. This is similar to the way the balloon should cut through the air. There are many thin balloons (that clowns use to make animals) that we could use for the experiment instead of big fat party balloons.
- A second possibility is to reduce the friction from the string and on the straw. The string that we used for this experiment was like cotton and had little hairs sticking out, which probably added to the friction that hit the straw. An easy fix to this type of string is to simply use better string to hang the machine from. These strings could be plastic wiring that can be found almost anywhere, as long as we use something that will not cause a lot of friction. At the same time, we can also make reduce friction by manipulating the straw. If we lubricate the inside of the straw that slides along the string with oil, honey or some kind of liquid that isn’t too thick or too watery, then the movement of the straw along the string will be very easy.
- Finally, we can transfer and manipulate the distribution of sound and elastic energy (the flapping part of the balloon) at the opening of the balloon. If we can manipulate both these types of energies, we can successfully transfer the energies to contribute to the thrust of the rocket, therefore the movement of the rocket. We can do this by inserting another straw into the mouth of the balloon and taping the outsides. Taping the outsides of the opening will control the flapping of balloon’s mouth and greatly reduce unnecessary energy transformation that will instead go to directing the balloon to moving one way. The flapping originally changed the direction of the rocket (the balloon only went one direction because it was stuck to a string) and was really trying to move the balloon in multiple directions and angles. This is why when you randomly release a balloon, it flies all over the place and doesn’t head in one direction. If the mouth is controlled and doesn’t flap all over the place, the direction will also be controlled and the movement of the rocket won’t be disrupted by forces trying to push it to go different directions. Also, by taping a straw into the mouth, the air will come out a lot more smoothly and there won’t be that ridiculous sound that comes from the mouth when a balloon is released. Without this sound, more energy can again be put into the thrust of the balloon which can push it further.
- Additionally, (as a bonus), putting a straw into the inside of the balloon will actually force use to different means of pumping air into the balloon. It will definitely be harder to blow air into the balloon if there is a straw taped into it during the entire experiment but if we use small bike pumps (which do exist), we could actually increase the accuracy of the experiment. This can be done, for example, by keeping track, of how many pumps of air you put through the straw.
In general, if we make sure to decrease the barriers that the rocket faces and redirect the transfer of energy to types of energy that will help the work done by the machine, we can be able to increase the efficiency of the machine.
We assumed that the balloon’s force while it ran along the string was 0.5N but we also knew that this measurement was wrong. After retrieving the actual information about the balloon’s mass, I was able to calculate the actual work and power of each balloon run and the actual average work and power.
I was able to calculate the actual information after finding the actual mass at 3 grams. If I convert this to kilograms, it would be 0.003kg. Afterwards, to find the force of an object of that mass, I’d multiply it by 10N. The actual thrust force of the balloon is therefore actually 0.03N. I was able to plug this value in to calculate for work and then use my previous information (of the total time of each run) to find the power. The values are shown above, along with the respective calculated averages next to the originally calculated average to compare with.
If we compare the average that we’ve calculated now (0.21 joules and 0.1 watts for work and power respectively), the difference is drastic and each value is far from the original averages (3.55 joules and 1.61 watts for work and power respectively). The drastic difference in the averages are proof that the assumed force was invalid, very off and inaccurate. This can also been seen in the fact that 0.5N and (the real force) 0.03N are also quite far from each other.