Why do we need to know the centre of mass of an object ? For which objects, its position may differ from that of the centre of gravity

Apropagating wave in space with electric and magnetic components. these components oscillate at right angles to each other. it may travel in a vacuum. determine the term.

Two charged particles, q1 and q2, are located on the x-axis, with q1 at the origin and q2 initially at x1 = 12.2 mm. in this configuration, q1 exerts a repulsive force of 2.62 µn on q2. particle q2 is then moved to x2 = 18.0 mm. what is the force (magnitude and direction) that q2 exerts on q1 at this new location? (give the magnitude in µn.)

An air-filled capacitor is formed from two long conducting cylindrical shells that are coaxial and have radii of 26 mm and 103 mm. the electric potential of the inner conductor with respect to the outer conductor is - 700 v (k = 1/4πε 0 = 8.99 × 109 n · m2/c2) the energy stored in a 1.0-m length of this capacitor is closest to