The most accurate reading of the length of a $6.28 \,cm$ long fibre is ............... $cm$.

What is the estimation of error? Write the methods for estimation.

Resistance of a rod is calculated by measuring its length with the help of a meter scale of least count $1 \ mm$. Its radius is measured with the help of a screw gauge having $50$ divisions on the circular scale and a pitch of $1 \ mm$. Resistivity of the material is exact. The length of the wire is found to be $20 \ cm$ and the diameter of the wire is $4 \ mm$. Find the percentage error in the calculation of resistance.

Two intervals of time are measured as $\Delta t_1 = (2.00 \pm 0.02) \ s$ and $\Delta t_2 = (4.00 \pm 0.02) \ s$. The value of $\sqrt{(\Delta t_1)(\Delta t_2)}$ with correct significant figures and error is

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?

The pressure on a square plate is measured by measuring the force acting on the plate and the length of the sides of the plate. If the maximum percentage error in the measurement of force and length are $4 \%$ and $2 \%$ respectively,what is the percentage error in the measurement of pressure (in $\%$)?