Monday, July 31, 2017

Physical Quantities: Quantities and Units

Where are we going with this? This page will assist in understanding the relationship between units and the quantities they measure.

When looking at the world around, people pretty much automatically attend to physical quantities, pretty much without thinking about them. The youngest child intuitively can judge differences with some skill.

By the time the child reaches school age, the following conversation would seem drastically out of place:

Teacher: "Mary, how far can you run?"
Mary: "I can run six pounds! Very heavy!"

Almost all children would know that far and heavy describe different physical quantities.

As most readers of this would know, one of the jobs of science is to complicate things. No… not that… 
to clarify things by assigning specific words with specific meanings to specific ideas, objects and concepts in order to make discussion precise and accurate (which are both words surrounded by confusion, ironically).

Therefore, with regard to physical quantities, a certain set of words are used to describe specific things.

Included in those sets of words are two sides to every quantity. 

One side is the concept of what is being described. Mary, in the dialog above, gave an answer about one quantity with words used for a different. 

The other side of the concept is, for every quantity, specific ways that it is measured (which are called units). Pounds do not go with a "how far" question!

First what are the things being described?

Types of Physical Quantities

The following list is certainly not all-inclusive. It includes a few very common and familiar quantities that are measured.

Distance: Distance is a measure of… okay, distance is so common it actually has different meanings, so in science, there are different words that are used so in order to be more specific.

Distance (Most General): The measure of how far two points are from each other, as in, "The tip of the antenna was 12 meters from the the surface of the window."
Distance (A specific case) Think Distance Traveled: The length of a path that a moving body takes, as in, "The dog ran from tree to tree through the park until it was at the tree next to the one where it started, covering a distance of 250 yards." Compare with Displacement, next!
DisplacementThe measure in a straight line between where a moving body begins and ends, as in, "The dog ran from tree to tree through the park until it was at the tree next to the one where it started, resulting in a displacement of only 4 yards."
Length, Width, Height: Pretty much what you expect, on this! The measure of how far specific points of an object are from other points on the object.
Other things that would be included in the distance concept are circumference, radius, perimeter, range, altitude, depth… You can probably think of others. But will you?

In general, distance is the measure of how far apart two things are.

Volume: (Not talking about sound, which borrowed from this concept for its own purposes!) This also is pretty much what you expect!

Volume is the space in three dimensions that an object or substance takes up (or holds). If a tank holds 25 gallons of gasoline, then the volume of the gasoline that filled the tank would be (duh) 25 gallons. A bottle of soda holds 20 ounces. A different bottle holds 2 liters.

Mass: Mass is a physical quantity that is the result of how many protons, electrons, and neutrons are all in a given space. It is actually not directly observable, though there are devices (scales and balances) that combine with gravity (or other acceleration) and other laws of physics so that it can be measured. Mass is a measure of the amount of matter an object contains.

If two things with the same volume have different masses, one would feel heavier than the other.

Good news: There are scales that measure mass, so you can just plop things down and get a number!

Weight: Weight is a measure of mass under a particular condition. Many people have heard things like, "Well, on the moon, I'd only weigh 12 pounds."

Weight is a basic concept that people are very familiar with. Weight is the degree of heaviness something has.

Time: This is something people very intuitively understand, but which is actually a very abstract concept. Stop reading and write down a definition of time. Not how time is measured! Try a definition of time that does not use seconds, minutes, hours, days, etc. and see what you come up with!

According to the Oxford dictionary, time is the indefinite continued progress of existence and events in the past, present, and future regarded as a whole. 

Time is a very basic concept in science, and fortunately, our intuitive understanding is enough for us to use it.

How do you measure these quantities?

The list below will connect the quantities above with the SOME OF THE units used to measure them. Common units in science are in bold and "normal" abbreviations for the common units are in parenthesis..

NOTE: Italics units are from the English system

Distance: inches, feet, yards, milesmeters (m), kilometers (km), centimeters (cm), light-years

Volume: gallon, ounce, cup, teaspoonliters (l or lt), milliliters (ml)

Mass: slugsgrams (g or gr), kilograms (kg)

Time: hours (hr), minutes (min), seconds (s)

A few more…

Some units are derived or combined from the basic units but are so common they are worth noting here.

Force: Newtons (n or N), Pounds
Weight (which is a force) is also measured in pounds, newtons
Speed or Velocity: MPH, m/s, cm/s, km/hr
Speed or Velocity is a distance unit divided by a time unit.

Acceleration: (m/s)/s, MPH/s, (km/s)/s, cm/s2  m/s2
Acceleration is a velocity unit divided by a time unit.

And also…

Temperature: Temperature is, within the kinetic theory of matter model, defined as the average kinetic energy of the molecules in a system or substance. How much thermal energy is present? How hot is it? The absence of thermal energy is described as being cold. Temperature is measured in degrees. In science, officially in Kelvin degrees (°K), but often in Celsius degrees (°C). 

SI Units

With so many units, things could get confusing. So… Some smart people made a decision that, a a general best-practice science would be built on seven base units. They also gave it a fancy name:

The International System of Units

Then… they decided they would abbreviate it… from French 

Système international (d'unités)

The seven base SI units and some corresponding constants allow meaningful and uniform dialog about all matters of science.