## 03453660 2 years ago cm, mm, inches, yards all are base units or derived???? please explain

1. akash123

base units...

2. imron07

i dont think there's base and derived unit

3. UnkleRhaukus

$0.9144[\text{m}]\equiv91.44[\text{cm}]\equiv914.4[\text{mm}]\equiv36[\text{in}]\equiv3[\text{ft}]\equiv1[\text{yd}]$

4. 03453660

@UnkleRhaukus i agree with you but mm,cm yards ,inches all were declared as base units by proffesor walter lewin from MIT.

5. UnkleRhaukus

what does base units mean ?

6. 03453660

base units are the units of base quantities eg lenght, time, mass etc

7. UnkleRhaukus

the SI base unit of length is the meter

8. 03453660

in SI can i consider millimeter(mm) as base unit???

9. UnkleRhaukus

nope $1[\text{mm}]=10^{-3}[\text m]$

10. 03453660

ok what about kilogram(kg) in SI system $1[kg]=10^3[g]$

11. 03453660

but still kg is considered as base unit for mass

12. UnkleRhaukus

yep, there are only seven Si base units

13. Vincent-Lyon.Fr

There are seven SI base units. http://www.bipm.org/en/si/base_units/ In case of doubts, always refer to the full SI official brochure: http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf

14. UnkleRhaukus

and i do agree that the base unit for mass is a confusing choice

15. Vincent-Lyon.Fr

cm and mm are decimal submultiples of the base unit "metre". As for the kilogram, the brochure states: "As an exception, the name of the kilogram, which is the base unit of mass, includes the prefix kilo, for historical reasons. It is nonetheless taken to be a base unit of the SI. The multiples and submultiples of the kilogram are formed by attaching prefix names to the unit name “gram”, and prefix symbols to the unit symbol “g” (see 3.2, p. 122). Thus 10−6 kg is written as a milligram, mg, not as a microkilogram, μkg."

16. 03453660

i asked from my physics professor that in SI system can i consider mm,cm,inches , as base units for length in SI system. he told me no they are derived units because these units are derived from meter. now i m confused that kilogram is a base unit in SI system so its also derived from gram then why not the gram the base unit for length

17. Gowthaman

I think.... The weight of 1 lit volume of water at zero degree Celsius is taken as reference for fixing base unit of weight at initial time, which is equivalent to 1 kg present SI unit. and 1 kg is easily feel-able weight comparing to 1 gm which is very less weight to feel...

18. 03453660

@Vincent-Lyon.Fr thank you so much sir.

19. Vincent-Lyon.Fr

@03453660 Derived units are defined as products of powers of the base units. E.g. m/s or kg/m³ So cm and mm are not derived units. Some of the derived units received special names: kg.m/s² is the coherent derived unit for force, and received the name newton, symbol N.

20. 03453660

so far i was concern about this issue was because i was in doubt because someone told me its base unit and others told me no its derived unit, but i was in strong belief that once a quantity is considered as base or fundamental quantity then all of its units are base units

21. Vincent-Lyon.Fr

This is because there was a confusion between "base quantities" and "base units". Length is a "base quantity", but among all units of length, only the metre is its "base unit".

22. 03453660

@Vincent-Lyon.Fr thus base units depends upon the system i use eg in SI system the base unit for mass is kilogram and in CGS system the base unit for mass is gram

23. Vincent-Lyon.Fr

Yep, I should have written: Length is a "base quantity", but among all units of length, only the metre is its SI-"base unit".