Author Options:

My five year old would like to know why the water whirlpools when he drains the tub? Answered

I need a simple answer to why the water whirlpools when you let the water out in the tub. I'm trying to find an answer that my 5 year old will understand. Thank you.


Ok...this one stumped me, not because of the science, but because of the age of the child... I did a little searching and found a book that *might just give you a good method for explaining this to your child in terms he can understand...even if it's not 100% accurate (conventional wisdom about this isn't the end of the story!)... Anyway. It may or may not be available in your library system, but (of course) it can be purchased at many online stores... the title is: Worms, Shadows, and Whirlpools: Science in the Early Childhood Classroom Best of luck. As Kelsey said, your kid is now a scientist, or at least has the makings for one...if he keeps it up, consider getting him involved with science programs at school.

And to those of you who have inquisitive girls, the same advise and comments apply

I used to be an inquisitive five-year-old girl myself :). (In fact, I dimly remember asking my father this question. He spent 10 or 15 minutes explaining it in terms of friction and angular momentum, including a (badly needed, if poorly understood) explanation of angular momentum; and I came away with the idea that "Well, all the water can't get down the drain at once - that only makes sense - and so that's what the water does about getting crowded into itself." Not sure whether his excess of scientific detail was stupifying, or planted the seeds for later understanding in greater depth, or both.)

Lol...so you were in this boat already!... vaguely similar situations...Asked grandfather about the origin of the stars when I was about 3-1/2 while riding in the back seat of his car at night on a freeway in Chicago..He said they had been there for millions of years and that they would expand and contract over and over forever...something happened in my brain that night...something about time and space and infinite...can't quite put it into words, but it was inspiring...and then there's my dad (now retired University Mathematic Prof) who tried to teach me some calculus when I was 7...i got it too...concept of dx/dt and speed, but that was as far as he got with that game, although it stuck fast.. but bows and arrows and bikes and toy cars and "stuff" effectively ran it right over until I was in my late teens (not that I didn't absorb stuff...just not higher math) I think these and others planted seeds for me, even if the concepts were what I would now consider "over the head" of a child...I was never a "normal" child. sounds a bit hypocritical of me I suppose, having said the stuff above in prelude to Worms, Shadows, whirlpools...

I guess I was, not that I'd given the matter any conscious thought for 40+ years. :)

I don't think you're at all hypocritical, rather that you were acting on the reasonable (both statistically and given the tenor of the Question) assumption that the Question Author's son is a "normal" child and the Author is a "normal" parent.

It takes a "non-normal" parent like my father to deliver a 15-minute science lecture in answer to a question like that, and I expect it takes a "non-normal" child to sit and listen to the whole thing. :)

I wish I could remember what it was I had asked (something about what Time was made of, maybe...?) that prompted the most inspirational thing my father ever said to me:
"We don't know, Pumpkin."

Not "I don't know" - I'd asked him about plenty of stuff he didn't happen to know before - but "We don't know."

I was 8 or 9 by that time; and the idea that there were things that the whole of humanity, the entire human race - adults included! - just didn't know was absolutely stunning to me. I could see that there was this vast pool out there, of the collective knowledge that humankind had amassed; and then this much greater, perhaps infinite, body of things that weren't known by anybody - some of which could maybe never be known, but some of which definitely could be eventually found out. And I got the idea then that I wanted to learn as much as I could of both types of knowledge.

I wan't say that the interim has been a steady and scholarly pursuit of that great ambition, but I've gotten at least as far as being able to tell a hawk from a handsaw - provided, of course, that the wind is southerly :).

well, ty for defending my honor ;) I kinda thought I was anyway...struggled with answering the question (eez a biggy imo) for a while inpart because of that...I tend to talk to little kids with a combination of TMI and baby talk, trying to work in their language and still presenting complex info in hopes that that they can absorb it...unfortunately I tend to use beeg words interspersed with the more childfriendly...On those few occasions I get to interact directly, I more often get a response of crossed eyes and "can I have a cookie?' Or "Play airplane now!" instead... except for one of my former girlfriend's daughters that is...sharp as a tack...maybe sharper. From the time I met her (age 4) she was able to comprehend a surprising amount of adult level conversation, even if she would sometimes respond with " you're boring me...let's play airplane!" Quiet, observant little girl overall, and smart as they come. Artistic, creative, and last I heard, will be entering gifted science program at her elementary school this year. Hawk, as in the plasterer's tool or the raptor?

Shakespeare Hamlet,II,ii (1600), Hamlet says: 'I am but mad north-north-west: when the wind is southerly I know a hawk from a handsaw'.

Never having heard of the plasterers' tool, I always thought Hamlet meant the raptor. But if Our Boy Bill was indeed thinking of the tool, that takes this line from being frustratingly absurd to being a nice little joke. (Any fans of the frustratingly absurd will still be able to enjoy almost everything else Hamlet says.)

I'm not familiar with the reference directly, although it *would be in keeping with Shakespeare's sense of humor to make that play on words. And it would make it make more sense

I'm embarrassed to admit my experience with Old Bill is in watching rather than reading...

Maybe an analogy to the effect that, since all the water molecules can't fit through the drain at once, it's their way of lining up to wait their turn; and the line/spiral/whirlpool moves as each one gets to go on through?

I'll bet that 5 year old will accept this answer until he gets old enough to understand the whole concept.

Thanks. :) I'm hoping so, if only because (while I can list the concepts and manipulate the equations well enough) I have yet to develop an fully intuitive understanding of angular momentum myself :)

That is a very nice analogy! It's more a statement about the incompressibility of fluid flow than about a conservation law, but it's still a very nice, and meaningful, visual image.

Thanks, Kelsey! :)
If I remember right, five is about the age when one is learning about lining up, waiting one's turn, and sharing (of space, time, and other resources) in general, so I thought it might be especially accessible.

(And I will submit, on behalf of the strict accuracy of my argument, that if the fluid were not incompressible (i.e., were infinitely compressible), then it could indeed all go through the drain at once; and no angular momentum would ever be developed or require conservation.

This would violate several other rather important laws of physics, of course. It also fails to address why the motion is a spiralling vortex rather than say, a linear seiche or a vertical "ant hill" effect - but then, my "lining up to go through" analogy doesn't really address that either.)

Done, and now your reply has no context.

ty. I apologize if I sounded harsh, but I've let thing like that go in the past to my detriment...made a pact with myself recently not to do so again. Have also deleted

Hi, I think that when the water drains into the plug hole small rough patches on the plug hole disturb the water flow. this makes one side go faster and so the water goes into a spin. Once spinning the flow of the water keeps it going. Try this: get a soda bottle of water over the sink up end it and see how long the water takes to drain out. - Re fill the bottle and this time put your hand over the open neck and give the bottle a swirl to make the water go round then remove your hand - the water will spin faster and drain out faster as well.

By the way, your five year old is now officially a scientist; that is an excellent and perceptive question, with a lot of subtlety hidden behind it. Give them a good answer, even if it seems too complicated to you. Let them work out an understanding on their own, asking you for help as needed.

It's not, despite urban legend, the Coriolis effect (the rotation of the earth). For the adults, it's a simple case of conserving angular momentum. A given packet of water is flowing toward the drain, but not necessarily directly along a line. The offset means that there's angular momentum, which must be conserved. As the packet gets closer to the drain center, it's forced to move faster, but also to (attempt) to maintain its distance, "orbiting" the axis of the drain. When you have a lot of packets of water doing that, the net result is a vortex flow (whirlpool).

For a five year old, the "figure skater" analogy may or may not work (do you have winters with ice skating in your area?). However, you can use the play merry-go-round as a model. Have him/her stand close to the edge of the merry-go-round while you get it spinning. Now have them walk into the middle. It'll "magically" start spinning faster! That's conservation of angular momentum. The water does just that by moving from out in the tup toward the drain, and the result is the same. Faster spinning and a vortex flow.

Often attributed to the Coriolis effect, see section "Draining in bathtubs and toilets". It's not an easy one to explain to a 5 year old. I think Gorfram may have your best answer.