Physics Behind the Collapse

Gravity, air resistance, collision, and metal fatigue

Gravity,  air resistance and metal fatigue all played a role in the collapse of the Towers.

Gravity- acted as an extra force that the already fatigued building had to work against to stay upright. The force of gravity is said to have caused the building to fall quicker than it would have without gravity (duh).
Air resistance- without air resistance, the building would have crashed to the ground at a much higher speed, causing a massive amount of damage.
Collision- the planes hit the towers at 945 km/h and 796 km/h. The collisions caused both towers to go up in flames and parts of the metal/steel frames to collapse.
Metal Fatigue- This is the MAIN reason the towers collapse. As the fires burned, the metal became extremely hot, turning red in some areas, this made the metal weak. There was also added strain on the remaining support structure because of the collapse structures where the planes hit. Eventually, gravity and heat caused the foundation and framework of the building (mainly metal/steel rods and bolts) to collapses, bringing the entire towers down with it.

                                In short:

1.Planes hit destroying parts of the framework
2. Towers catch fire
3. Metal structure heats up and becomes incandescence (becomes so hot it's red)
4. Metal is unable to support the towers
5. Bye Towers