I’ve recently had my first experience rowing in a canoe and I noticed that when I finish rowing, there is always 1(sometimes more but usually 1) spinning moving hole in the water. it usually gets filled up with water nearby after a few seconds but they sometimes last around half a minute. this phenomena seems to happen when you put an object in a liquid, move it and then scoop it out of the water(tried with soup). is there a reason why that happens? maybe some well known physic stuff.
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Those are called “vortices” and occur both in liquids and gasses. To understand why this occurs think about what happens when an object moves through the water.
Water in front of the object, an oar blade in this case, needs to get out of the way. Some is pushed along with the blade but it can’t compress the liquid very much, and gravity prevents it from moving upward too far either. The blade also leaves a void in the water behind it which is going to be filled by nearby water flowing in. That water in turn leaves a void to be filled by other water, so water in front of the blade wants to flow around the side and behind it.
That movement starts up a circular flow, the momentum of which persists for some time. That is the vortex you see as the “water hole”.
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