Science | Science Experiment of the Week 4 | 245 - Rolling Bottles

Friction - Fill a bottle with dirt and a bottle with water. Which one rolls farther?

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This Week's Experiment - #245 Rolling Bottles

These experiments are from Robert Krampf - The Happy Scientist

This week's experiment is a fun one and may be surprising. Before you read the "why it works" part, be sure to try it and see if you can figure out what is going on and why. That is the real fun of science. You will need:

2 soda bottles with screw-on caps
water
sand or dirt

Fill one of the bottles with water and put the cap on tightly. Fill the other with sand or dirt. Don't tell my wife, but since it is raining outside, I used some of her potting soil. Put the cap on that one too.

Now for the fun part. Find a long hallway or room with lots of open floor. Roll the dirt filled bottle. Notice how far it rolls and the way that it behaves. Then roll the water filled bottle. Try to use the same amount of push to get it going. Does it roll the same distance? Does it stop in the same way?

OK, now stop and don't read any more until after you have tried this and thought about it. Did you try it? Really? I didn't think you had. Now go try it. I mean it! Go do it, now.

Now that you have tried it........wait a minute. That wasn't long enough for you to do the experiment. Stop reading this right now and go do the experiment.

Have you done it? Promise? OK, then we can go on. What happened? The bottle with the dirt rolled farther. It gradually slowed down and finally stopped. The bottle with the water started rolling just fine, but it slowed down very quickly instead of gradually and did not roll as far.

Why? The bottle with the dirt acted like a solid. As it rolled, all of the dirt rolled with it. It behaved just as we would expect it to. With the bottle of water, things are very different. When you roll the water bottle, the bottle rolls but the water in the center does not. Friction causes some of the water along the sides of the bottle to move with it. The friction between the bottle and the water, and then between the spinning water and the water in the center slows the bottle and stops it from rolling. You can test this by swirling the bottle before you roll it. This gets the water spinning so that it will all move with the bottle. In this case the bottle behaves much more like the one full of dirt. The water is moving along with the bottle, so there is less friction. Be warned that if you swirl the water in the wrong direction, there will be more friction and it will stop even faster than it did unswirled.

OK, now that you have read it all, you can go try it. I knew that you would not be able to wait until after you did the experiment.

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