Why do we need units?
As soon as you start learning about physics, you are quickly introduced to units. In day to day life, you might say you weigh 50 kg or 3 hours have passed. You know units are those sometimes abstract words, and sometimes named after scientists, that come after some numbers.
As you might know that we humans only needed to define seven basic units to represent seven fundamental quantities. All other physical properties can be given units derived from these fundamental seven units. They are units of :
Length, Mass, Time, Electric current, Thermodynamic temperature, Amount of substance and Luminous intensity
(Find out what units they have.)
Importance of measurement
At the heart of physics, is measurement. Think about this. To study different phenomena, we need to measure the quantities involved in it (even if theoretically). For example, if we want to study motion, we need to measure distance(length) and time. If we want to study gravity, we need to measure mass. If we want to study optics, we need to measure luminous intensity in some cases. These are just some examples. Think about studying motion without measuring distance and time. You cannot do it. It just becomes meaningless without measurement.
And once we figure out the importance of measurement, we then come across another problem. To measure anything, you quickly have to set a definition of ONE amount of that thing. That is a unit. The alternative meaning of unit is single or one. You may know that from mathematics. Imagine you want to measure the size of your room. And then you want to measure the distance between your two eyes. You cannot keep changing units. You come to the conclusion that you need some basic standard single measurement by which you will measure both the distance between your two eyes and the size of your room.
So what units are just the definition of how much a single amount of something to be measured measures. Does that make sense?
Think of your ruler you use in geometry classes. They have short divisions in centimetre and millimetre and sometimes inches. They are the divisions of length. The standard of length across the world is set as the meter and all others are derived from it. Other units have also been defined in this way.
The single unit of measurement for all the seven physical properties are arbitrary and chosen by humans and are not some basic property of nature.
Different magnitude of units
So now you know that units are important to define what a single entity of something to be measured is and then how much we are measuring against this set standard. You may also wonder why we have so many units for just one quantity, for example, length. centimetre, meter, kilometre, miles, inches, light years, etc.
This is because if you go around measuring things, you will quickly find that measuring everything with a 1meter set as standard will be really hard. If we have to measure the width of a paper, then it is very tough to do so with a 1-meter scale. You then have to divide this one-meter scale into parts and then measure against these divisions how much the paper is wide. These divisions that you made are nothing but alternate units for 1 meter. You then again decide to make one single small division of one meter to be something (ie a centimetre) and thus a new unit is born that is, in essence, nothing but the division of one meter. Similarly, you can come upon the definition of one kilometre or even one light year as multiplication of one single meter.
Before the world was so connected, humans in different geographical regions used different units for their measurements. Famous recent examples are pound-kilogram, inches-meters unit difference used in the USA and Europe/India. This is a stark evidence that units are arbitrary and our own consideration for ease of study of nature. Do a task: Go around and measure different quantities, be it the length of things, or mass or time span for a week without using any units. You will quickly realise the importance of having some units.
Units have other tiny advantages while solving mathematical equations. We want you to research this advantages on your own.