Iceman,
You are not taking in to account that the chair begins to accelerate once it starts to fall. The math is complicated but to better illustrate I will use a falling person as an example. A falling human will reach terminal velocity after about 15 seconds which is 195 kmh or 120 mph. After 8 seconds it would reach 90% of terminal velocity and after 3 seconds it would reach 50% of terminal velocity. Obviously, that chair thrown from that height would seriously injure anyone that it hit.
Some information:
Average mass of plastic chair:
6kg - can find actual mass and beacuse I said so
Assumed height of floor thrown from:
90m at 3m per floor 30th floor
https://twitter.com/DanGreks/status/1095038267781799936
Assuming no drag force, chair never reaches terminal velocity - very bad assumption.
Assuming no horizontal force is applied.
Okay...
I will use work done, all the gravitational potential energy is converted to kinetic and thus the net work done is the whole of the gravitational potential energy the complete transfer of energy. Work done is defined as the cross product of the applied force and displacement. In the ideal chair throw it's perfectly vertical so the angle is zero...
W=F*D*cos(0)
= F*D*1
=F*D
Work done must be equal to gravitational energy...
W=Egpe
F*D=m*g*h
F=m*g*h/D
I suppose it must be stated that the "D" is how far the chair travels into the thing it hits (aka you). So to make this easy to calculate we can assume it travels 1cm (0.01m) before you notice. (Perhaps a bad assumption - but fuck it).
Anyways, time to plug and chug.
F=(6kg)(9.8m/s^2)(90m)/(0.01m)
=529200N
Das a lot of force
Comparable to;
The average force applied by seatbelt and airbag to a restrained passenger in a car which hits a stationary barrier at 100 km/h.
So if it is needed to be said, death is just about certain.
https://en.wikipedia.org/wiki/Orders_of_magnitude_(force)
I spent to much time on this and am deeply ashamed, but I will not let the effort go to waste.
Please correct me if anything is wrong, or if you can make it more accurate I encourage you.
Thanks.
PS (if you appreciate this, seriously hook me up with the chair girl contact information, I really want to book a session.)