# Rolling a half full water bottle and simple harmonic motion

1. Aug 6, 2011

### sungj25

hi im just saw this phenomenon and see if there is any links to the topic that im learning in school.

So when i rolled a half full (depends on how optimistic you are ) water bottle with a certain amount of force, the bottle went back and forth for a little while before stopping.
Is this simple harmonic motion? and does it affect the rate of oscillation (wavelength? im not sure) if I change the volume of the water in the bottle?

Oh and can anyone explain why it happens? i think its something to do with momentum right....?

thank you, and have a nice day! (:

2. Aug 6, 2011

### Fewmet

Welcome to Physics Forums, sungj.

First, I want to note (as a teacher) that's a wonderful observation. Noticing something like that and want to explain it is a wonderful mindset. To carry on with that approach, I encourage you to experiment. Is the water doing anything interesting when the direction reverses? Does it happen with an empty bottle? Can you tell if changing the amount of water has any effect?

I tried to get a bottle to do and and could not. I tried two plastic bottles and one glass one. Can you describe the bottle you used?

3. Aug 10, 2011

### sungj25

Thanks for the compliments (:
um I tried with a normal plastic bottle and a soda can, they both worked (but i had less than half the bottle of water).
I also noticed that it also works with other materials. I tried it with beans and the bottle still oscillates around a fixed point before stopping.
oh and I was really lucky and got the permission to use school equipments, so I might use a data logger to conduct a proper experiment for this!
I'm thinking of changing the mass of the water in the bottle to see how it affects the frequency of SHC...
Basically I'm going to roll the bottle from an inclined plane (to control the force/velocity? because i think it has the same potential energy.. I need more advise on this..). When the bottle finish rolling the inclined plane, I will let it continue to roll on the flat surface and set up the data logger from a certain distance and record the displacement change against time, giving me some sort of sine curve on the graph.
But when I was doing the background research I noticed there are way more factors that I have to consider, such as rotational kinetic energy and moment of inertia, that I never thought about. (but these are beyond my cover... haha)
If possible, I would like some advises that can improve my experiment.

PS: even though this experiment is beyond my knowledge level, I think I will still enjoy it (:

4. Aug 10, 2011

### A.T.

Push it harder. The effect depends on the initial acceleration.

5. Aug 11, 2011

### Fewmet

Those are reasonable things to try. Be sure to run many trials and to really think in advance about what your data might look like and what it might tell you. Are you just messing around with it to see what you can find out, or are you trying to answer a specific question (like "Is this simple harmonic motion")?

I don't think you have to take rotational kinetic energy into account. It will only be the rotational kinetic energy of the bottle, and the walls of the bottle are probably so thin the bottle rotating so slow that the energy will be too small to worry about.

You might want to test A.T.'s assertion that the phenomenon depends on the initial acceleration and see ig you can figure out why.

This is an engaging phenomenon that I think my students will also want to investigate. Can you post here about what you find out?