This Week's Experiment - #245 Rolling Bottles
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.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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