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Why do we use head as a measurement of a pump's capacity and not pressure? The short answer is that
it is more convenient. Convenient for who? The pump manufacturer and also the user. The main reason
is that head is an easy measurement to make; in the case of static head it is a vertical measurement
from one level to another.

Head is a term that has a chameleon like qualities, it has a secret identity. It can be interpreted
as the height of a body of water which is its normal day to day appearance or it can reveal itself
as a form of energy. Let's take a look at its units. If it is equivalent to a height then its unit
must be in feet or meters.

For example

Let's take look at another way to express those units. If head is really a form of energy there must be an energy term associated with it.

We know that ft-lbfs is an energy term.

The secret identity of head is that it is really energy per unit weight which happens to be the same unit a feet which is why we can turn it into a measurement.

This is significant because we can add and subtract energies just as we do with measurements. We can't do that with pressure measurements and learn anything useful about our pump system.

So what is energy? It's all around us in our day to day activities. If you're a linebacker running down the field for a touchdown you are a moving mass with velocity, if the defense hits you head on it has to come up with the same amount of energy to halt you in your tracks. Everyone has thrown a ball; if you get hit by a ball at 40 mph you feel the energy. Electric bills, you are charged per kilowatt-h, that's energy. The engine in your car might be 150 hp, of course horsepower is power, and that's energy per unit time, the more energy you can summon in a short period of time the more power you have.

We use energy terms in our pump system such as head because we need to know how much energy will be required to do the job (that's the energy the pump requires) such as raising liquid at a height and overcoming friction.

Let's show how this works with a familiar example of a cyclist at the top of a hill. A cyclist has potential energy at the top of a hill even though she is not moving; as she rolls down the hill she is exchanging her potential energy for kinetic energy or velocity energy. As she reaches the bottom of the hill all her potential energy has been converted to kinetic energy neglecting friction for the moment. That's the principle of conservation of energy, if one increases the other must decrease, there is no free lunch.

The formula for potential energy is:

m is the mass, g the acceleration due to gravity, the product of these two is weight or W and h is the height above a reverence plane. Why a reference plane? Usually when you measure heights on large structures you refer to a reference plane such as sea level, you then subtract your elevation above sea level of the start point from the elevation of the lowest point to establish the height difference.

Let's put some numbers on this, if the rider weighs 150 lbfs and the bike weighs 20 lbfs and the hill is 50 feet high then the potential energy is (150 + 20) x 50 = 8500 lbf-ft, according to the formula above the kinetic energy will also be 8500 ft-lbf.

The formula for kinetic energy is:

In the Imperial system (yes, I know sorry) we need a conversion constant gc to make the units work.