Question

a heavy box is placed on a tabl. the box has a weight, or force, of 450 newtons. explain why it doesn't fall through the table.

Answer 1

I think reading up on a certain Newton Law will help you on this one

Answer 2

When the box is on a table it is constantly applying its weight as a downward force on the table. But Newton third Law states "When two bodies are in contact, the forces they apply on each other are equal and opposite"
Thus we know that the table is also applying a contact force as an upward force which balances the downward force acting on it. Since the forces get balanced, there is no motion in this scenario.