Three rods of same mass are placed as shown in figure. The co-ordinates of centre of mass of the system are

A charge is uniformly distributed on the surface of a spherical rubber balloon. As it is blown up, the total electric flux coming out of the surface

Resolving power of a telescope can be increased by increasing

The ratio of the total energy of the $2^{\text {nd }}$ orbit electron for the hydrogen atom ($^1\mathrm{H}$) to that of a helium ion ($\mathrm{He}^+$) is :