If I have an object in my hand that weights 1.5 kg and I throw it from the second floor, how do I calculate its weight when it reaches the ground?

I need this answer to know what kind of capacity of a net should I install under an under construction building so that the items or labors that fall will reach a zero incident.

Thank you

An objects weight dioesn't change simply because it's falling. You say the object "weighs" 1.5Kg, but that's actually its mass, not weight. Its weight is mass times acceleration due to gravity =- 1.5 Kg x 9.8 m/s^2 = 14.7 newtons. It weighs 14.7 newtons whether measured on the 2nd floor or on the ground.

I suspect that what you really meant to ask was how much force should the net be able to withstand. Since F=ma, you will need to first determine the decelaration of the objects when they hit the net. If you can tell us how much the net will deflect when the object lands you can then determine its deceleration, and from that the force imparted on the net.