A planet is moving in an elliptical orbit around the sun. The work done on the planet by the gravitational force of the sun

(i) is zero in no part of the motion.

(ii) is zero in some parts of the orbit.

(iii) is zero in one complete revolution.

(iv) is zero in any small part of the orbit.

Which of the following is true?

A steel rod has a radius of 10 mm and a length of 1 m . A 80 kN force stretches it along its length. If the Young's modulus of the rod is $2 \times 10^{11} \mathrm{~N} / \mathrm{m}^2$, then the change in length is

A block of iron contains a hollow cavity as shown below. The block weighs 6000 N in air and 4000 N in water. If the density of iron and water are $6 \mathrm{~g} / \mathrm{cm}^3$ and $1 \mathrm{~g} / \mathrm{cm}^3$, then the volume of the cavity is (assume, $g=10 \mathrm{~m} / \mathrm{s}^2$ )

A heating element of mass 100 g and having specific heat of $1 \mathrm{~J} /\left(\mathrm{g}^{\circ} \mathrm{C}\right)$ is exposed to surrounding air at $27^{\circ} \mathrm{C}$. The element attains a steady state temperature of $127^{\circ} \mathrm{C}$, while absorbing 100W of electric power. If the power is switched Off, then approximate time taken by the element to cool down to $126^{\circ} \mathrm{C}$ will be (neglect radiation)