Science Buddies: "Ask an Expert"

My daughter completed the science buddies experiement about how will different sizes of balloons with different volumes of air affect the lenght of time a balloon powered hovercraft will hover. Her teacher stated that she has 2 independent variables, I do not understand - what would have been her independent variable?

I need to know a bit more about your experiment to fully understand it. What was the hypothesis? What was the test proceedure? What trials were performed?

Independent variables are ones that the experimenter has control over. For example, if your daughter selected/chose different sizes of balloons to test with, then the size of the balloon is an independent variable because it was something that was changed by choice. If your daughter decided to fill the same balloon with different volumes of air, then the volume of air is an independent variable because it was changed by choice. Because it is ususally difficult to draw conclusions when more than one thing changes, it is ususally best to construct/design experiments so that there is only one independent variable (one thing that is changed by choice). The hypothesis should be based on predicting what happens if the independent variable is altered.

If you provide more information, I can better help you to see if some subset of what you tested is a single independent variable and if your hypothesis is appropriate (e.g. related to something that is measurable and is based on your independent variable(s).).

Not sure if I'm going about this the correct way, but Craig answered my first question and here is the information that he needed to respond more accurately. (The teacher stated she had 2 independent variables, not quite sure what she means)

My daughter's hypothesis was: I believe, that of the 3 balloons I am going to test, the balloon which is largest and holds the greates volume of air will cause the balloon powered hovercraft to hover for the longest period of time.

She performed 3 trials with 3 different size balloons, a 7 in balloon, 9 inch balloon 12 inch balloon. She filled each balloon with different volumes of air and tested their hover times. Each size balloon was tested 5 times.

I hope this help you - thank you for your help!!!!

If I understand your testing, I agree with the teacher's concern about being able to draw any scientifically supportable conclusions.

She performed 3 trials with 3 different size balloons, a 7 in balloon, 9 inch balloon 12 inch balloon. She filled each balloon with different volumes of air and tested their hover times. Each size balloon was tested 5 times.

From you first statement: 3 trials with each of 3 different sized balloons would mean 9 test runs with 9 test results. From the last statement, each size balloon was tested 5 times. I'm still confused.

In order to reduce "random variation", scientific testing methods should attempt to repeat the same exact test case multiple times. For this grade level, that is usually means a minimum of 3 trials with the same conditions.

Did she fill the same balloon with the same volume of air three times and measure the results? If not, then setup and measurement errors were not controlled and the repeatability of the experiment is unknown.

How did she determine that she had the same starting conditions for each of the trials in each test condition?

With air, mass, pressure, temperature, and volume are all related. My experience with balloons has been there is a "memory" of past stretching. The first time you blow up a balloon, it takes more pressure to start blowing it up. For the next few inflations, it gets easier each time until at some point there isn't much of a difference between inflations. How did she eliminate this varriation?

If you want to draw conclusions from tests run with different balloon sizes, you need to eliminate all other differences from the tests. I'm not sure that this can be done. If you attempted to do this by keeping the volume of inflation air the same with different balloons, you would probably have different initial pressures. If you attempted to keep the initial pressures the same, then you would have different initial volumes.

The hypotesis

I believe, that of the 3 balloons I am going to test, the balloon which is largest and holds the greatest volume of air will cause the balloon powered hovercraft to hover for the longest period of time.

has a basic testability flaw. I could put very little air in the two largest balloons and compare them to the smallest balloon filled almost to the breaking point and probably disprove the hypotesis. In short, I can probably alter the test conditions to make any of the three balloons the best or worst in terms of hover time.

To the dad doing the project:
How did you get the balloon to hover? My son and I did the exact same project and constructed the hovercraft exactly the way the instructions on the website instructed; however, our hovercraft did not hover. ??Help please?