Class 8: Approximations (Lecture Notes)

Approximations

Significant Figures: The number of digits in a measurement about which we are certain plus one additional digit which is uncertain are known as significant figures.

Thus, in $52.58$ cm, there are four significant figures. Similarly, $10.434$ have $5$ significant digits.

Rules for Finding the Number of Significant Figures

Rule 1: All non-zero digits in a measurement are significant.

Example: $-546.13$ g has five significant figures. $307$ m has three significant figures.

Rule 2: In a decimal number which is greater than $1$ , all digits (including zeros) are significant.

Example:

$205.002$ kg has six significant figures.
$63.00$ has four significant figures.
$2.50$ has three significant figures
$92.0501$ km has six significant figures

Rule 3: In a decimal number which lies between 0 and 1, all zeros to the right of the decimal point but to the left of the· first non-zero digit are not significant

Example: $0.00686$ km has only $3$ significant figures while $0.001010$ kg has $4$ significant figures.

Rule 4: If the measurement is a whole number, then the case becomes ambiguous if there are zeros to the left of an understood decimal point but to right of a non-zero digit.

Rule 5: All the final zeros in a decimal number obtained by rounding off a decimal to a given number of decimal places are significant.

Example: when $7.896$ is rounded off to two decimal places, we get $7.90$ , which has three significant figures

Rounding off a Decimal to the Required Number of Significant Figures

Sometimes during measurements it is required to obtain the value of a decimal number correct to the required number of significant figures.

Example: Round $74.312$ kg to three significant figures:

Solution: The measurement $74.312$ kg has $5$ significant figures. To round it off to $3$ significant figures, we are required to round it off to $1$ place after the decimal. Therefore $74.312$ kg = $74.3$ kg rounded off to $3$ significant figures.