Why can't we lift objects while sitting?

[Hepic]

What happened about inside forces? For example If we are "out" of a chair,we can lift that,but If we are sitting we can not lift that.
I know that we can not "fly",so that is a reason,But I need physics reason.
Thanks!

 

[Drakkith]

Because you are applying an upwards force on the chair but an equal downwards force on yourself and right back into the chair, cancelling out the upwards force. In comparison, when standing on the floor, you apply an upwards force on the chair, but a downwards force through yourself and to the floor! So the forces on the chair don't cancel and there is a net upward force, which raises it.

 

[Hepic]

Thank you!