So the other day, I devised this thought experiment:
Consider an infinite tunnel, and you drop a coin of mass $m$ into it. Considering the effect of gravity to be applicable and neglecting air drag and other viscous forces, it seems that from Newton's second law the coin would accelerate indefinitely since force ($mg$) is constantly being applied. But on the other hand I also know nothing can go faster than speed of light, as stated by Einstein's theory of relativity so how can I describe the above experiment where it seems like the coin would approach the speed of light?
I read somewhere else that when a object made to accelerate indefinitely then as per Mass-Energy Equivalence the mass of object begins to increase exponentially and hence greater and greater force is required to increase object's speed.
But somehow the above explanation doesn't seem to explain much in my thought experiment.
I would like to know the actual reason why the above scenario can't happen.