**Physical quantity variants**

The magnitude of everything that can be *measured o*ver time, length, mass, area, volume, density and strength which are variants of **physical quantity**.

The fundamental physical quantity cannot be defined or expressed from others. For example, the time, length and mass are **primary physical quantities** that can be described in terms of more basic or primary. Whereas the units defined in terms of two or more basic or fundamental units called derived units.

- The area is a derived quantity which is described in terms of two lengths. In SI, the unit of area is the square meter (m
^{2}). - The volume is a quantity that is described in terms of three lengths. The unit of volume is the cubic meter (m
^{3}). - Density is a derived quantity that is described in terms of mass and volume. The unit of density in the SI is kg / m
^{3}.

For example, the area and volume are described in terms of length, the density is described in terms of volume and mass, and the force is described in terms of mass, **length and time.**

**Fundamental physical quantity: TIME**

Time is a fundamental quantity. The subjective notion of time is not reliable, so you need to measure to determine reference moments. A reference time can be considered as zero time or origin of time.

For example, to measure the time needed to move from our house to the school; the starting time will be the time zero or origin to start measuring time.

To measure time intervals needed to reference events repeat at regular intervals. Samples are; the rotation of the Earth on its axis, the motion of the Moon around the Earth and the motion of the Earth around the Sun.

Every day we use different types of watches that are designed precisely to indicate equal time intervals.

**Fundamental physical quantity: LENGTH**

Length is a critical magnitude related to the distance in space from a point which is taken as reference. Any point in space can be considered as the origin of the distance to be measured.

For example, when we measure the distance between two points a paper sheet, we place a ruler so that the zero of the scale coincides with one of the points. We consider a distance from zero and measure the distance.

**Derived physical quantity: AREA**

Area (A) of a surface is a derived quantity and described in terms of length. Area units are square units since they are the result of multiplying two units of length.

**The area of a parallelogram** **formula** (squares, rectangles, rhombuses and rhomboids) equals its base multiplied by its height.

**The area of a triangle** **formula** is basically the half of its base multiplied by its height.

**Area of circle formula**: Considering the circle as a regular polygon of an infinite number of sides, its area can be calculated using the same formula that is used in regular polygons. That is:

** **

**Derived physical quantity: VOLUME**

Volume (V) of a body is a derived quantity that is described in terms of length. Volume units are cubic units, since they are the result of multiplying three units of length.

In general, the volume of a regular prism either (when straight and their bases are regular polygon) equals the area of one of its bases multiplied by its height.

To calculate the **volume of an irregular solid** indirect measurement procedures are used. One of the most used methods is; introducing the body whose volume is to be measured in a graduated cylinder with a given volume of water: submerging the body, the volume of liquid displaced is equal to the volume of the body.

**Fundamental physical quantity: MASS**

Mass (m) is a fundamental physical quantity related to the amount of matter in a body. All bodies around us are made up of matter and therefore have a mass. The air, the water, the sun, the earth, all living things and all the bodies that exist in the universe are made of matter and, consequently, all have a mass.

The mass determines how to change the speed of a body when a force is applied. If the same forces act during the same time on bodies having different masses, the speed change is less for the greater body mass.

**Derived physical quantity: DENSITY**

*Density* (D) is a derived **physical **quantity that relates to the mass and volume of a body. If they had several bodies of the same volume but different mass, it is said that their densities are different.

Similarly, if you have several bodies of the same mass but different volume, then we say that their densities are different.

To illustrate this, suppose you have 10 cm ^{3} of iron, knowing that it’s mass is 78 grams.

Then 1 cm ^{3} of iron has a mass of 7.8 grams, i.e., the density is the equivalent of dividing the mass of a body by its volume.

Density is a characteristic property of each substance. For example, pure iron objects made of any mass or volume always have the same density.

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