# Water pressure question

I turn on the tap in my kitchen, and put my hand just beneath the opening, and feel the water pressure on it. Then I move my hand down to the bottom of the sink, and the water pressure seems to be much greater. Why is this? I see that the stream is slightly narrower toward the bottom, but not enough to explain the difference.

I’m thinking that it’s because of gravity. Obviously gravity has little to do with the water until it clears the tap. Then, on the way down, it starts accelerating, causing it to fall at a greater speed, increasing the pressure on my hand.

I can understand this with regard to a waterfall; in Niagara Falls, the water at the top looks almost gentle, compared with the water beating the rocks below. But can acceleration explain the water in my sink, with such a short distance?

I’m thinking there aren’t many other forces at work here from which to choose an explanation.

But how can the difference be so great over such a short distance?

You would think it would be the opposite…ie, pressure being greater closer to the source rather than farther away.

My WAG: falling objects accelerate until they reach their terminal velocity. IIRC, it accelerates at 9.8 m/s/s, so a half-second fall will make the water at the bottom 4.9 m/s faster than the water at the top.

Edit: Christ, I mean, in a vacuum it’s 9.8 m/s/s, in a vacuum. But the point remains - falling objects accelerate to a certain speed, so the very act of falling makes the water accelerate. The only time it’d slow down is if the water was already coming out faster than terminal velocity. But I know just enough about hydrodynamics to know that it is mad ruddy complicated.

I would think the aerator makes a big difference. Since the air is added into the stream at the source rather then as it delaminates on it’s way down. If you took the aerator off (but kept the GPM the same) maybe there would be a difference.

Minor nitpick, but the PRESSURE is the same, atmospheric; about 14.7 Lbs/sq in. what you are feeling is the FORCE of the water, which is a matter of the volume of water, it’s density, and it’s speed.

Now, the volume is the same, since all you have is what comes out of the faucet. The density changes if you have an aerator faucet. Immediately after leaving the faucet, the water is mixed with air bubbles. Some of this dissolves in the water, some of it reaches the surface and break, joining the atmosphere, and some remains as bubbles one foot from the faucet. That which dissolves and that which joins the atmosphere reduces the apparent density, so the density is greater one foot from the faucet than it is immediately after leaving the faucet.

The speed increases, too, as has been mentioned, due to the acceleration of gravity, at a rate of 32 ft/sec/sec. This is probably the major contributing factor to the effect you observe, since the water will be moving about 2 ft/sec faster one foot from the faucet than the speed it left the faucet with (I’ve had a few beers, so that may not be entirely correct, but it’s my best guess).

excavating (for a mind)

Interesting question. Long ago I reasoned the smaller cross section was due to the higher speed. I never though about the loss of air. You get the same effect from a hose without an aerator or even pouring out of a cup.

As you lose air, you lose mass even if you gain density. Therefore I think losing air will reduce the force on your hand at the bottom.

There may be some aerator effects, but the loss of diameter generally is due to the water’s acceleration. Visually, it’s like a group of skydivers crammed into the door of an airplane. A group of 10 can exit the plane within one second but they’re instantly stretched out so that there’s, say, 10 seconds between the first and last one. Gravity makes the water fall faster and hit with more force at the bottom, but it’s also hitting a smaller point which probably amplifies the effect a little.