Rubidium-87 was the first and the most popular atom for making Bose–Einstein condensates in dilute atomic gases. Even though rubidium-85 is more abundant, rubidium-87 has a positive scattering length, which means it is mutually repulsive, at low temperatures. This prevents a collapse of all but t...

The original question I was asking was Why a body does not rotate if force is applied on the centre of mass? The obvious answer is that Torque ( R x F) is zero at as R is zero at centre of mass, but that defeats the purpose of my question. Why is centre of mass so special that we take torque abou...