Explain why Carbon dioxide is effective at extinguishing fires?
I would suggest obtaining the force needed empirically. The force can be considerably reduced simply by denting the side of the can. Best material would be steel being strong and cheap.
Prop it on fire resistant bricks
Depends upon why you were fired. If it was for misconduct, you're hosed. If it was simply because you could not do the job or if you were laid off due to a lack of work you'd be fine.
