Science Buddies: "Ask an Expert"

In The experiment Knock the Bottles Down what is the manipulated variable?? Please Help thank you!

Hi,
The manipulated variable is the "thing" that you are changing in the experiment. In this experiment, you are changing the "center of mass" of the bottles so that's your "manipulated variable."

Good Luck and please post again if you have any other questions.
Willey

Thank you for the reply! I know it’s the center of mass but how am I changing the center of mass in each of the pyramid formation by the amount of water that is added or taken away from the bottles or is it the positioning of the bottles.my teacher wants specifics. Thank you

Hi,
That's great. You are on the right track with this then. When you remove / add / re-arrange water bottles and its content, you are shifting the position of the center of mass. You should be able to calculate this position based on how the bottles are arranged.

For example, starting off with just one bottle that's completely filled, the center of mass will be somewhere in the middle. (It may be slightly above / below depending on the shape of the bottle). Now, if you have bottle that only half filled, the center of mass will be closer to about a 1/4" of the way from the bottom. With this, you can then examine how the center of mass shifted in the different combination that you tested.

Just as an another point of resource, please take a look at this for how to calculate the center of mass:
https://www.khanacademy.org/science/phy ... er-of-mass

Good Luck and Have a great weekend!
Willey

Thank you so much for your reply it was very helpful. I have another question. We had to do a trial with all three bottles were empty. Where would the center of mass be for this pyramid formation.

Hi,
In the case of the three empty bottles, the bottle themselves are now the "source" of the weight. (Technically, you still have air in the bottle but I am going to suggest that it won't matter into your calculation since the weight of the air is so much smaller than the weight of the bottle). At any rate, you should be able to use symmetry to get to Center of Mass of the individual bottle. Then, please use the same method / analysis for the 3 full bottles to determine the center of mass for the pyramid. (For the actual mathematical derivation of the center of mass, you will need to use Calculus/integrate the "hollow cylinder" - unfortunately, this is not something I can easily explain in this forum format.)

Good Luck!
Willey