Four persons measure the length of a rod as $20.00 \ cm$,$19.75 \ cm$,$17.01 \ cm$,and $18.25 \ cm$. The relative error in the measurement of the average length of the rod is:

The International Avogadro Coordination project created the world's most perfect sphere using Silicon in its crystalline form. The diameter of the sphere is $9.4 \,cm$ with an uncertainty of $0.2 \,nm$. The atoms in the crystals are packed in cubes of side $a$. The side is measured with a relative error of $2 \times 10^{-9}$,and each cube has $8$ atoms in it. Then,the relative error in the mass of the sphere is closest to (assume molar mass of Silicon and Avogadro's number to be known precisely)

In the expression for time period $T$ of a simple pendulum $T = 2 \pi \sqrt{\frac{l}{g}}$,if the percentage error in time period $T$ and length $l$ are $2 \%$ and $2 \%$ respectively,then the percentage error in acceleration due to gravity $g$ is equal to ......... $\%$

The potential difference across the ends of a conductor is $(30 \pm 0.3) \ V$ and the current through the conductor is $(5 \pm 0.1) \ A$. The error in the determination of the resistance of the conductor is: (in $\%$)

Why is the error in a measurement expressed with both positive and negative signs?

The outer and inner radii of a hollow cylinder are $(4.23 \pm 0.01) \text{ cm}$ and $(3.89 \pm 0.01) \text{ cm}$ respectively. What is the thickness of the wall of the cylinder?