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Sunday, August 16, 2009

What is a physical quantity?

Physical quantities

A physical quantity is a property of an object that can be measured with a measuring instrument.For example length, width, time, weight, etc.

It is important for you to be able to say which instrument is used to measure which physical quantity.The table below shows a few examples of physical quantities and the instruments used to measure them.

Physical quantity Instrument

Length                                                        Metre rule

Temperature                                            Thermometer

Time                                                             Stopwatch

Volume                                                       Measuring cylinder

Of course as we go through the different chapters we will be able to introduce more physical quantities and their measuring instruments.

Basic and derived quantities

Quantities can be divided into two types :

1. Base quantities

2. Derived quantities

Basic quantities

Basic quantities are the fundamental quantities that are not related to each other and that are use to derive all other quantities.

There are seven basic quantities. They are

1.  length

2.  time

3. mass

4. Thermodynamic temperature

5. electric current

6. amount of substance

7. luminous intensity

Derived quantities

Derived quantities are just quantities that are derived from one or more basic quantities.

For example area is a derived quantity because it is derived from the basic quantity length.

Area = length * length

Volume is a derived quantities because it is derived from the basic quantity length.

volume = length * length * length

density is a derived quantity because it is derived from length and mass, two basic quantities.

density = mass/(length * length * length)

As you can see all other quantities apart from the 7 basic quantities are derived from the seven basic quantities and it can easily be shown as in the example above.

Can you show what are the basic quantities that are used to derive the following derived quantities?

a) force

b)  kinetic energy

c) power

d) pressure

I would give the answer after a few of you would have give your answers.

Good luck and see you next time.


  1. best lecture for today.........

  2. good answers

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  4. gud but there should be more description

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  6. very good that's nice

  7. Thanks, I really need to understand more about physical quantities.

    This post really helps.

  8. very nice....thanks for the post

  9. this help me alot thanks for the post

  10. it great to learn something new

  11. sir, i have a query.. colour is not a physical quantity even it depends on frequency which is a physical quantity

  12. As you have said vikas colour is not a physical quantity since it cannot be measured. However it is closely related to the quantity frequency.

    Similarly some properties such as obesity can be associated with mass but still not be the same.

  13. excellently described in simple words

  14. It's Gud but should more elaborated


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