Why fossil fuel has been used more in the existing world?

Lets say that an object of a certain volume has a certin mass. what happens to its mass when you double its volume?

Starting with only the balmer series light (visible light), how could we ensure that the solar panels generate a current that mark can use for his power station? a)by gradually increasing the brightness (amount) of light that we shine on it. b)by gradually increasing the frequency of the light we shine on it. c)by gradually increasing the wavelength of the light that we shine on it.

In addition to gravity, what is the other predominant force that affects the motion of a pendulum?