Question

Four children measure the length of a table which was about $2\;m$. Each of them used different ways to measure it.
(A) Sam measured it with a half metre long thread.
(B) Gurmeet measured it with a $15\;cm$ scale from her geometry box.
(C) Reena measured it using her hand span.
(D) Salim measured it using a $5\;m$ long measuring tape.
Which one of them would get the most accurate length? Give the reason for your answer.

Answer 1

Hint: Four children used different methods to measure the length of the same table. Accuracy of measurement is a measure of how close the measured value is to the actual value of the physical quantity. The accuracy can be increased by measuring using high precision instruments.

Complete answer:
Sam measured the table using a half metre long thread. To measure the $2\;m$ long table using a half metre long thread, he has to measure it four times. He cannot measure the table in one go. And also there are no markings of measurement on the thread. We cannot consider the thread as a standard measuring instrument. Therefore there is a chance that Sam’s measurements are not correct.
Gurmeet measured using a $15\;cm$ scale. Compared to the $2\;m$ long table, $15\;cm$ is very small. Gurmeet cannot measure the table in one go. The chances of errors in measurement are very high in the case of Gurmeet.
Reena measured with the span of her hand. The span of a hand cannot be taken as a standard measurement device. The span of the hand varies for different people. There is no standard length for the span of a hand. Hence Reena’s measurements are also not accurate.
Salim measured using a measuring tape of $5\;m$ length. He could easily measure the table in one go using the tape of $5\;m$ length. Since a tape is a standard measuring instrument of length, the measurement will be more accurate than the measurement taken by the others. Hence we can say that the most accurate measurement will be made by Salim.
The answer is: Salim made the most accurate measurement because he used a standard measuring instrument that could measure the length in one go.

Note:
When we use an instrument to measure a quantity, the measured value will be different from its true value. The difference between the measured value and the true value is known as the error. The errors in measurements are broadly classified into systematic errors and random errors.