This is another problem from my childhood which I like very much. By an irresistible cannonball we shall mean a cannonball which knocks over everything in its way. By an immovable post we shall mean a post which cannot be knocked over by anything. So what happens if an irresistible cannonball hits an immovable post?
Before you look at solution of this problem you may suggest your your opinion in comments.
This occurance is an impossiblity. The irresistible cannonball can be described as an object that would impart an infinite force on anything it struck. Force is defined by mass*acceleration. In order for the force on the cannonball to be infinite, the mass or the acceleration must also be infinite. If the mass is infinite the cannonball is actually a black hole, an object for which the physics are quite hazy. If the acceleration is infinite, the cannonball would approach the speed of light, and the cannonball would quickly use up all available energy as it accelerated with diminishing returns.
Assuming the post is an object that will neither move nor break when struck by any force, it must be an object of infinite density, and therefore infinite mass. As we noted before, an object of infinite mass is a black hole (or the universe, assuming the universe is infinite). So one of two things will happen in this situation: the two objects are black holes, and black holes have only limited individuality. On colliding, they will simply (not so simply really, it’s actually quite an involved process) merge with each other. Or the other situation, in which the irresistible cannonball has infinite acceleration, and consumes all the energy in the universe in it’s efforts to accelerate with diminishing returns as it approaches the speed of light, leading to the heat death of the universe.
Even given any of these extreme physics situations, it’s unlikely any of these forces could be accurately catagorised infinite.
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