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Physics 1: Physical quantities

A Biomedical science student, certificated in english, spanish, french and japanese language, who also writes diverse topic articles.

physics-1-physical-quantities

Content

• What are quantities and physical units?

• Fundamental units of the International System.

• Prefixes used in the International System.

• Conversion factors between systems of units.

• Example of conversion factor.

• Extra material.

What are quantities and physical units?

A physical quantity is part of a physical system that is capable of quantification, wich means, it can be measured. When taking a measurement, a certain physical quantity is compared to a standard reference or patern; this standard defines a unit of quantity. An accurate and reliable measurement requires a consistent system of units: the most widely used system of units by scientists and engineers is the International System of Units.

Other physical quantities are obtained by relations between the fundamental quantities; some examples of derived physical quantities are velocity, acceleration, or force.

Since physical quantities can have ranges of variation of several orders of magnitude with respect to the fundamental unit, it is very common to use prefixes and scientific notation to express multiples or fractions of the fundamental units.

Fundamental units of the international system

PHYSICAL QUANTITYSYMBOL OF THE QUANTITYFUNDAMENTAL UNITYSYMBOL OF THE UNITY

Length

l, r, x, others.

Meter

m

Mass

m

Kilogram

Kg

Time

t

Second

s

Thermodynamic temperature

T

Kelvin

K

Electric current intensity

I, i

Ampere

A

Amount of substance

n

Mol

mol

Luminous intensity

Iv

Candela

cd

Some units derived from the international system

PHYSICAL QUANTITYSYMBOL OF THE QUANTITYFUNDAMENTAL UNITYSYMBOL OF THE UNITY

Speed

v

-----------

m/s

Acceleration

a

----------

m/s2

Force

F

Newton

N

Energy

E, K, U, others.

Joule

J

Potency

P

Watt

W

Negative Prefixes used in the I.S.

This prefixes are used with rational numbers, like 0.1, 0.01, 0.001.

FACTORNAMESYMBOL

10 (-1)

Deci

d

10 (-2)

Centi

c

10 (-3)

Mili

m

10 (-6)

Micro

μ

10 (-9)

Nano

n

10 (-12)

Pico

p

10 (-15)

Femto

f

10 (-18)

Atto

a

10 (-21)

Zepto

z

10 (-24)

Yocto

y

Positive Prefixes used in the I.S.

FACTORNAMESYMBOL

10(1)

Deca

da

10(2)

Hecto

h

10(3)

Kilo

k

10(6)

Mega

M

10(9)

Giga

G

10(12)

Tera

T

10(15)

Peta

P

10(18)

Exa

E

10(21)

Zetta

Z

10(24)

Yotta

Y

Here are some examples of the use of Prefixes

  • 1000 m = 1 km.
  • 1000000 bits = 1 Mbit
  • 1 MW = 106W
  • 1μs = 10−6s.

Conversion factors between systems of units

There is another system besides the I.S. It is the English System with different units that have their equivalent in the international system.



Length

ENGLISH SYSTEMI.S. EQUIVALENCY

Inch (in)

1 in = 0.0254 m

Foot (foot)

1 foot = 0.3048 m

Yard (yd)

1 yd = 0.914 m

Mile (mi)

1 mi = 1 609 m

Mass

* The ton of the English system is 907.2 kg, different from the one used in the international system which is 1000 kg

ENGLISH SYSTEMI.S. EQUIVALENCY

Pound (pounds)

1 pound = 453.6 g

Ounce (oz)

1 oz = 28.35 g

Ton * (t)

1 t = 907.2 kg

Volume

ENGLISH SYSTEMI.S. EQUIVALENCY

Gallon (gal)

1 gal = 3.785 L

Quart (qt)

1 qt = 946.4 ml

Cubic foot (ft3)

1 ft3 = 28.32 L

Example of conversion factor

  • To convert from inches to cm we take the factor of conversion (1 inch = 2.54 cm) and multiply by the name of inches we have.

⇒3 inch= (2.54 cm/1 inch)

Inch and inch is canceled, while we keep cm, that is the unity we want to change into.

⇒ 3(2.54/1) = 7.62 cm

  • Other examples are:

1gal = 3.785L

⇒ 2gals= (3.785 L/1gal)

⇒ 2 gals= 7.570 L


1 oz = 28.35 g

⇒(1000g1kg/1kg)(1 onza/28.35g)

⇒ (1000)(0.03527) = 35.27 onzas


© 2021 Daniela Alejandra Rodríguez Cerda

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