When is the total momentum of a system conserved?

When the net external force is zero

Explanation:

We can answer to this question by referring to Newton's Second Law, which can be written in the following form

where

F is the net external force acting on a system

is the change in momentum of the system

is the time interval

When the total momentum of a system is conserved (so, it does not change), its variation is zero:

In order to satisfy this condition, we see from the formula that we must also have

F = 0

so the net external force acting on the system must be zero.

which piece of furniture will have the most inertia and give the furniture movers the most difficulty in moving?  

answer

a 75kg dining table