Physical quantities are properties or measurements that we can observe and measure in the physical world. There are two main types: base quantities and derived quantities.
When you measure the length of your desk with a ruler, the length is a physical quantity.
Base Quantities or Basic Quantities #
Base quantities are the basic measurements that we use to describe the physical world.
Examples: #
- Length: Measuring how tall you are.
- Mass: Weighing a bag of apples.
- Time: Checking how long it takes to walk to school.
Derived Quantities #
Derived quantities are made by combining base quantities by multiplying them or dividing them. For example, speed is made from length (distance) and time.
Examples: #
- Speed: Calculating how fast you are running by dividing the distance you ran by the time it took. (example: 10km/hr)
- Area: Finding the area of your classroom by multiplying its length by its width. (example: 10m2)
SI Units #
SI Units (International System of Units) are the standard units used that are used to measure physical quantities.
There are a total of 7 base/basic quantity. Each one has its own symbol and its own specific SI unit.
Volume #
Volume is the amount of space an object takes up. For regular shapes, you can calculate volume with simple formulas.
For example, the volume of a cube is: Length × Width × Height
Density #
Density is the amount of matter (mass) that is inside a particular space.
We know that all things are made of matter (whether they are solids, liquids, gases). We also know that all matter takes up or uses space.
Density tells us how much matter can be stuffed into a particular space.
The image below shows that, in the same amount of space, gases are less packed together that liquids and solids. So we say that gases are less dense than liquids and solids.
Not all gases take up the same amount of space. Some take up even less space than others.
The same goes for liquids and solids. You can have some liquids that are more dense than other liquids (meaning that one liquid has more particles packed tightly together than another liquid, in the same amount of space.
How do we calculate density.
The symbol of density is an old Greek symbol that looks like the letter p We can also just use the letter D
From the formular above, we can see that Density (p) is calculated by dividing the mass of an object (mass is like the weight of an object) and the volume of that object (remember that volume is the space taken up by the object)
