I was in my 20s, studying engineering at the time. And from a physics standpoint we (my class mates, teachers and i) couldn't believe how quickly those buildings collapsed.
They fell at the acceleration of gravity. So if you dropped a hammer from the top of the building it would of landed at the same time the top of the building did.
This is impossible as there should of been a resistance below the airplane impact point that would have decelerated the collapse.
According to the laws of physics, the only possible way those buildings could of collapsed like that was if there was nothing holding it up. But obviously the building itself is holding it up.
So then how did they fall so fast.
We concluded, from a physics and engineering standpoint, that it was impossible for the plane damage to bring down the towers that way. Their had to have been demolition charges through the buildings core for it to fall that fast. And if you look at the destruction of a building using demo charges, it falls exactly the same way.
Without charges the top of the building may have crushed the levels that were damaged by the plane but would never have destroyed the whole building. At the very least there should of been a deceleration leaving the lower levels in tact.
This analysis is purely from a physics and engineering perspective.