@Cyoteblack Scientifically, the spear pretty much came from the outer space if you consider that height here on earth and it is a free falling body with that much height, thus that speed is logical. Though, even with that force, the impact shouldn't be that big due to the form of the object.
@joda1000 What doesn't add up is the distance travelled by the spear upward, considering it was dropped and neglecting the resistance as well when it goes up, it shouldn't be that high. Well, it is a different matter if the spear was thrown upward rather than dropped. As I computed, it needs roughly 9 minutes for the spear to reach the ground from the point it stopped/highest point as per height stated in the manga. Also, with that height and speed, that spear made of metal should be pretty much melted at some point, depends on the material.
@Ribath They're don't give a shit about it because it is a fantasy and with a touch of magic on top of that, so logically speaking don't need to touch that aspect, that negative gravity. What they're talking about is the numerical values of that certain event and the effect of that attack.