Liquid and Hydraulics

Hi, welcome to this video lesson about the behavior of liquids and hydraulics.

Mechanical comprehension exams often test on your knowledge of laws of pressure regarding liquids and how that is harnessed to produce mechanical systems such as hydraulics.

First, there are some general rules of physics to know, these are simplified, as the tests usually don’t require you to account for minute variables.

          Liquids are incompressible.

          Liquids perfectly conform to their containers.

          Pressure applied to an enclosed liquid is equally distributed throughout it.

Another thing to know is the equation for pressure. Most simply, pressure is equal to Force/Area. For water specifically, there is about .43 lbs of pressure per square inch for every foot of water depth.

Thus, the amount of pressure at the bottom of a 12ft pool is approximately 5.2lbs per square inch.

Given the rules above and what we know about pressure, let’s look at a few examples.

Here is one that displays the basic function of a hydraulic system. On one side is a vat of liquid which is connected by a hose to another vat. If the left vat is pressurized to move the liquid level down 1 inch, how much will the water level in the other vat rise, assuming they are the same size?

Because of the rules that we mentioned before, especially because pressure applied to an enclosed is equally distributed, when the liquid in the left vat is pressed down one inch, the water level in the right vat will rise one inch.

Another example. Here are two vats again. The left one is half the size of the one on the right. In this case, if the pump on the left moves down two inches, the one on the right will move up one inch. That’s because of the mechanical advantage is such that only half the force needs to be used on the left, but it needs to be pushed twice as far.

Let’s try a practice problem.

Imagine a diver practices diving at a 20ft depth in a diving pool. Where is the water pressure greatest?

   a. Where the diver is

   b. Halfway between the bottom and surface

   c. It does not matter, water pressure is always equal.

The answer is “a” because water pressure increases by .43lbs per pound if you remember. “b” is wrong because the surface has almost no pressure relative to the depth of 20ft. “c” is wrong because water only equally distributes pressure when it’s in a enclosed and pressurized space, like a hydraulic tube.

One more practice problem.

Two identical enclosed containers of liquid are connected by a pipe. If one side receives 12 lbs of force, how much will the other side receive due to the connecting pipe.

    a. 1 lbs

    b. 6 lbs

    c. 12 lbs

    d. unknown

The answer is “c”, because the containers are equal in size, and they are pressurized, they will distribute weight equally.

I hope that helps. Thanks for watching this video lesson.