# What is a physical quantity?

When studying physical bodies, phenomena, processes, it is necessary to somehow record their characteristics (properties, signs). At the same time, the characteristics can be not only qualitative (for example, the body falls, the gas expands, etc.), but also quantitative (at what rate the body falls, what volume the gas occupied, what mass the body has, what length of the trajectory, etc.).

Physical quantities are used to describe the quantitative properties of physical objects, phenomena, and processes.

So the physical quantities are length, width, height, mass, volume, speed, time, acceleration, area, etc.

Physical quantities in each case have their own values. So the speed of one body can be 10 km / h, and the speed of another can be equal to 20 km / h. Here we have the same physical quantity – speed. However, its meanings are different. Thus, the same physical quantity can describe different physical bodies, phenomena and processes.

In physics, each physical quantity introduces its own designation, most often expressed in one English letter. At the same time, there are cases when in different sections of physics the same letter can mean different things. For example, v denotes velocity and V represents volume.

The value of physical quantities is always expressed in some units of measurement. In other words, the value of a physical quantity is a named number. So the mass can be expressed in kilograms, grams, tons, etc.; time – in hours, minutes, years, etc.

Throughout history, different countries and parts of the world have formed their own units of measurement. So, for example, in our country it is customary to measure the distance in kilometers, and in the United States – in miles. In order to avoid confusion, the International System of Units (SI) was introduced. It is used in physics when measuring the values of physical quantities. In the international system of units, length is measured in meters, time in seconds, mass in kilograms. The designation of these values is respectively “m”, “c”, “kg”.

It happens that the value of a physical quantity is very large or very small. In this case, divide or multiply by 10, 100, 1000, etc., and a special prefix is added to the name of the unit of measurement. For example, 1000 meters = 1 kilometer, 1000 grams = 1 kilogram. The prefix “kilo” is used here. But in “60 seconds = 1 minute” each dimension has its own name (this is an exception).