hello in this video I will talk about physical quantities. The aim of this video is to share about how to properly convey physical quantities such as length mass or time in a clear and convenient manner so that it is easy to tell it to someone else and also easy for the person to understand. In singapore’s O’ level physics syllabus and for all other physical sciences we make use of an internationally agreed system of units called the SI units. For those of you who are curious about why isn’t it called IS unit, it is because as it is French and stands for (French: system of units). In this system of units, there are seven base quantities and yes these are all we need to describe the physical quantities of our world. These base units each at the own corresponding SI unit. We have length which has a base unit in meters, mass in kilogram, time in seconds, electric current in ampere, temperature in Kelvin, the amount substance in mol and luminous intensity in Candela. At this point in time, you may be wondering about quantities such as area, speed and density. Well, these are actually derived from the same base quantities that we have seen earlier. Come, let’s take a look at some examples of derived quantities. Take note that some of these derived quantities such as force, pressure and energy have special SI units. These are named after individuals who have contributed significantly in their field. These special units can also be expressed in terms of the same base SI units that we came across earlier in this video. It is quite common for more complicated derived quantities to have SI units to be substituted by names While SI units help to distinguish the different physical quantities that we encounter, there will be times when we need to talk about physical quantities that are either very large or very small compared to the SI unit because of this we may find ourselves writing a large number of zeros which is inconvenient and easy to make mistakes and this is where prefixes come in. here some of the common prefixes that you encounter in in your O’level syllables. they come in different powers of 10 and it is easy to convert between different prefixes by multiplying the differences in magnitude between them. For example if we want to convert 10 kilograms to grams we can multiply by 1000 which is the magnitude carried by the prefix kilo. Following the same logic converting from grams to kilograms can be done by dividing by 1000 Last but not least always remember to express physical quantities in terms of its numerical value followed by its prefix, if you intend to use it, and its SI unit Well that’s all for this video thank you for watching.