The centre of gravity(C.G.) of a body is defined as the point about which the algebraic sum of moments of weights of all the particles constituting the body is zero. The entire weight of the body can be considered to act at this point, howsoever the body is placed.

2b.i critical angle, in optics, the greatest angle at which a ray of light, travelling in one transparent medium, can strike the boundary between that medium and a second of lower refractive index without being totally reflected within the first medium

Precautions taken to ensure accurate results
i) Clean the ends of the connecting wires by sand paper.
(ii) The connections should be tight.
(iii) Close the key (K) only when readings are to be taken.
(iv) Zero errors and the ranges of the ammeter and voltmeter should be noted.

Q3xi.
Precautions taken to ensure accurate results
i) Clean the ends of the connecting wires by sand paper.
(ii) The connections should be tight.
(iii) Close the key (K) only when readings are to be taken.
(iv) Zero errors and the ranges of the ammeter and voltmeter should be noted.