How to PHYSICS:estimation and units

ESTIMATION BY ORDERS OF MAGNITUDE

In physics ,we deal with the measurement of quantities which vary in their magnitude to a large extent .Generally ,it is not  easy to express  and memorize the values  of some quantities.Therefore ,for convenience ,the magnitude of some physical quantities are expressed in the powers of ten .For this,we should remember that

10^0=1

10^1=10

10^2=10*10=100

10^3=10*10*10=1000

1/10=0.1

1/10*10=0.01

1/10*10*10=0.001

and so on

A quantity of magnitude greater the ten is expressed as positive powers of ten ,while a quantity of magnitude less then 1 is expresses as negative powers of 10

  By expressing the quantity in powers of ten we can easily estimate the order of magnitude of the quantity .Therefore ,a physical quantity having a large number of digits is expressed in powers of ten with one digit to the left of decimal 

physical quantity =M*10^n

where M is greater than 1 but less then 10 ,and n is a positive or a negative integer .The power of ten is called the order of magnitude of  the physical quantity and M is called its numerical value.

If M=3.2 ,it can be written as 10 or 10^10^0.5 which when rounded ,becomes 10^1.Hence if M<3.2 ,the power of 10 will be less the of unit of measurement

The Order of magnitude of a physical quantity is its magnitude in powers of ten when the physical quantity is expressed in powers of 10 with one digit to the left of the decimal .

Need of unit of measurement 

Physics,like other branches of Science experimental study which involves measurements .For a measurement of a physical quantity ,we consider a constant quantity of  same natures as a standard and then find the number which expresses, how many times the standard quantity is contained in the given physical quantity.

Year :one year is defined as the time in which the earth completes one revolution around the sun.The period of revolution of the Earth around the sun is 365 days 

1yr= 365 days =365*8600s=3.1536*10^7 s

Leap year: A leap year is the year in which the month of February is of 29 days .A leap is thus of 366 days .

Decade: A decade is of ten years.Thus,

1decade =10years=3.156*10^8s

Century: A century is of 100 years .In a century,there will be 24 years each of 366 days and 76 years each of 365 days .Thus,

1 century =100 years

                =(24*366+76*365)days

                = 36524 days =3.16*10^9s.

Millennium:A millennium is of 100 years . 

                 =3.16*10^10s.     

GUIDELINES FOR WRITING THE UNITS

Conventionally following rules are observed while writing the unit of a physical quantity:

• The symbol for a unit ,which is not named after a scientist, is written in lower letter.For example,symbol for metre is m, for second s,for Kilogram  is Kg and so on.
• The symbol for a unit,which is named after a scientist,  is written the first letter of his name in capital letter.For example, N for newton, J for joule, W for watt ,Pa for pascal ,Hz for hertz , C for coulomb.
• The full name of the unit,even when it is named after the scientist is written with a lower initial letter e.g.,unit of force is written as newton and not as Newton; unit of energy is written ad joule and not as Joule; unit of power is written ad=s watt and not as Watt and so on.
• A compound unit formed by multiplication of two or more units is written after putting a dot ,cross or leaving a space between two symbols.e.g,the unit of torque is written as N.m or N0*m or N m
• Negative power is used for compound units which are formed by dividing one unit by the other