Pressure in liquids [replacing lesson Pressure in liquids]

Let’s do an experiment! Take a plastic bottle. Using a nail or similar, make three identical holes, one above the other, distanced equally apart: one hole in the middle, one above, half-way between the top and the middle, and one below, half-way between the middle and the bottom. Tape over the holes. Fill the bottle with water.

Now, peel off the tape. Observe what happens. The water flows out through the holes in little streams. You can mark how far from the bottle each stream reaches. Is there any difference between the flow of water from each hole?

The topmost stream is slow, and doesn’t reach very far. The middle stream reaches a bit further. The bottommost stream comes out fast and reaches further away from the bottle than the other two. You might notice that as the bottle empties and the water level inside drops, all three flows start to lose speed and flow closer to the bottle. Finally the water streams begin one by one to stop squirting out of the bottle.

There is clearly some connection between the distance of the holes from the surface and how far the streams reach. In our experiment, gravitational force acts on the water, pulling it downwards. When a force spreads out over an area, this creates pressure. Now, by every hole, draw lines on the bottle. This way we can imagine the water in the bottle in different layers.

First, let's look at the line by the top hole. There is only one layer of water above it, so only one layer being pulled down by gravity. The layer of water exerts a small force, which means the pressure is low. But by the middle hole, there are two layers of water above. Two layers of water weigh more than one, so the gravitational force here is larger.

Pressure is higher. Above the third hole are three layers of water. So the gravitational force is even larger and the pressure is the highest. More water means more weight and a larger gravitational force, therefore - more pressure. The pressure exerted by the liquid increases as the distance from the surface increases.

The further a point is from the surface of the liquid, the more pressure is created by all the water above that point. But if the gravitational force is acting downwards, why doesn’t the water just drip straight down from the hole? Think of a water balloon. If you press it on one side, its other sides expand. Water is a liquid.

Apply pressure to a liquid from one direction, and the liquid moves around, pressing out in all other directions. The same happens with the water in our bottle. When gravitational force presses the water downwards, the water in the bottle wants to move in other directions. It presses on the bottle’s sides and pushes through the holes. The more pressure there is from above, the more pressure there is to the sides of the bottle as well.

The deeper you go, the higher the pressure in all directions. And the higher the pressure, the further the stream reaches. Would the pressure be different if the container were another shape? Let’s see… Take two differently shaped containers and make holes at exactly the same height in both of them. Fill both containers to the same level.

Is there any difference between how the water flows? As long as both containers are filled with the same liquid, the same distance from the surface means the same pressure. No matter what the shape of the container!