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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.

21 comments:

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

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  2. good answers

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  4. gud but there should be more description

    ReplyDelete
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  6. very good that's nice

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

    This post really helps.

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

    ReplyDelete
  9. this help me alot thanks for the post

    ReplyDelete
  10. it great to learn something new

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

    ReplyDelete
  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.

    ReplyDelete
  13. excellently described in simple words

    ReplyDelete
  14. It's Gud but should more elaborated

    ReplyDelete

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