Please help me with this physics problem?

Are we sure it's the mortar that causes this and not the accelerations due to gravity?. Without going into the mathematics at this stage, I'm thinking that if you jump off a chair not a lot happens to you, if you jump off a high rise building - well, think you can guess the rest. So why! Well you are 'falling' falling (accelerating) for longer if you jump off a high rise. Now consider the chimney, is not the top part falling for longer than the bottom part, and noting that concrete and mortar have good compressive strength but poor shear resistance. Could be that forces at the top part, due to gravitational acceleration, exceed those lower down as the chimney is falling, hence the break-up of the chimney part way up. Perhaps I'm mis-understanding the question, but suggest this is something to consider.