Three Holes

Three Holes

This week's experiment will give you some good practice at thinking scientifically. The experiment itself is very simple, but as with many simple things, the more you think about it, the more you will see.

To try this, you will need:

- a two liter soft drink bottle
- pliers
- a sharp nail
- water

First you need to empty the two liter bottle. I highly recommend using the soda to experiment with making the perfect ice cream float. Once the bottle is empty, rinse it and remove the plastic label if it has one.

drawing

We want to make three holes in the plastic bottle, each at a different depth. Holding the nail with the pliers, make a small hole about one inch up from the bottom of the bottle. I found that twisting the nail back and forth as you push makes it easier to start the hole. You want the hole to be round and as smooth as possible. Be careful not to tear the plastic (or your skin!) Once you make the hole, wiggle the nail around a bit to help make the hole round.

drawing

After you make the first hole, make the second about half way up the bottle, and slightly to one side. You want all three holes to be as close to the same size as possible, and you do not want the holes to be directly above each other. The third hole should be about an inch or so below the point where the top of the bottle starts to narrow, and again a little to one side. The photo on the right shows approximate locations.

drawing

Place the bottle in the sink, under the faucet. Fill the bottle with water, and leave the water flowing just enough to keep the bottle full as the water flows out of the three holes. We want to compare how far each stream of water goes, which is why we did not want one hole directly above another. We want to be able to see the three streams easily. Now you should understand why we are working in the sink, to keep from making a mess.

drawing
 

Before you actually try this experiment, take some time to think about it. Once the bottle is full, water will be flowing out of all three holes. If you measured how far each stream of water moved away from the bottle before it hit the bottom of the sink, which stream of water would hit the farthest away? Which stream would hit the closest? Why? Keep in mind that there may be more than one thing to consider in how far the water reaches.

Once you have spent enough time thinking to have a good idea of what will happen, turn on the water and try it yourself. Next week, we will look at the results, and figure out why it worked the way it did.

Have a wonder-filled week!

Go to Part 2.

Anonymous wrote on Mon, 05/06/2013 - 18:07:

I dont think that if you use a sharp nail that the bottle will open.

brevard wrote on Thu, 10/11/2012 - 16:43:

meowwww!!!!!!!!!!!!!!!!!!!

Anonymous wrote on Mon, 03/12/2012 - 02:40:

it is a very interesting experiment

Kim Aaron wrote on Wed, 03/16/2011 - 14:04:

While it is true that the water will come out of the bottom hole the fastest, it also has the shortest vertical distance left to fall before it hits the horizontal surface (i.e. table) that the bottle is standing on.

With various simplifying assumptions*, the (horizontal) velocity of the water coming out of the hole equals the (vertical) speed of an object free-falling in gravity from the surface of the water down to that point on the bottle where the hole is.

If the top hole is exactly 1 inch below the surface of the water and the bottom hole is exactly 1 inch above the bottom of the bottle, amazingly, the water coming out of both holes will move out exactly the same distance by the time they hit the horizontal surface on which the bottle is standing.

A hole exactly in the middle (halfway between the water surface and the table) will hit the table the farthest out.

*Here are some of the simplifying assumptions I made.
gravity is constant over the height of the bottle.
air pressure is constant over the height of the bottle.
water is incompressible.
water is inviscid.
the water comes out of the hole exactly horizontally.
there is no vena contracta effect.
air drag on the water stream is zero.
there is no cavitation.
There are probably other assumptions I made without even realizing.

Here's the equation I came up with for the horizontal distance:

y = 2 SQRT(H z - z^2)

where y is the horizontal distance at impact, H is the height of the water surface above the table, z is the vertical depth below the surface at which the hole is. y is maximized when z = H/2.

Anonymous wrote on Sun, 03/13/2011 - 13:20:

You made me think !
I think time taken to freely fall from a height has only square root dependance on height.
While the initial velocity provided by the pressure difference increases proportionally with depth.
u.t will be the distance. Hence the bottom one will go further i suppose.

Anonymous wrote on Thu, 03/10/2011 - 12:47:

If you have kiddos who are old enough to be around hot things, it is very easy to make holes in this kind of plastic with a soldering iron. It's like drawing in butter. You could poke three very nice, even holes in three seconds.

zdexter wrote on Thu, 03/10/2011 - 01:24:

Although I am not able to try it I do have a guess as to what will happen. I have done it before though. It is really hard to say what will happen. But I can think of several factors that affect it. First is air pressure. But there is approximately the same amount of air pressure above each of the holes. The second is the water pressure. The lower down in the bottle one goes the greater the amount of pressure the water above will be exerting simply because there is more water above it, so more weight. Another is the height of the holes above the bottom of the sink. The higher the hole is the greater amount of distance the water can reach away from the bottle before it hits the ground/sink. The size of the hole also makes a big difference. However since the holes are all the same size this will not have a big effect on the experiment and distance the water will go. And finally gravity has an effect. But again it is approximately the same for all holes.

That is all the factors I can think of for now though there are probably others. Putting all of those together I think it should be that the lowest one will go the furthest by a little then the next highest will be next and the top one will go the shortest distance. This is because the bottom hole has the most water pressure on it but it is also the closest to the ground. The next highest hole has less pressure and more height and the top has the least pressure but most height. The water pressure however has the greatest effect on the distance the stream of water shoots out. But because the others have more height they will be close to the same distance.

Anonymous wrote on Wed, 03/09/2011 - 18:59:

I do this every year when we do observations and inferences. I cover the holes with electrical tape, put the lid on, and ask kids to predict what will happen when I uncover the top hole. They are AMAZED when no water comes out.
Then we uncover the others one at a time, and they start asking "what would happen if" questions. So we have a great time playing with the bottle. They love when I hold it horizontally with the holes on the bottom and still no water comes out. Then I loosen the lid and make a water fountain. They come to the conclusion that water won't come out until air goes in. It's so much fun!

Anonymous wrote on Wed, 03/09/2011 - 18:15:

Not gonna try it, but I say that the bottom one will go further as it has more water above it than the rest of the holes and the pressure that water makes is what makes the water go further.