Analysis
The same PHB pages 147 - 148:
An improvised weapon includes any object you can wield
...
If a character uses a ranged weapon to make a melee attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage.
This passage gives us the definition:
- Any object being used as a weapon which is not normally a weapon (which you can wield)
- A melee weapon being used as a thrown weapon
- A ranged weapon being used as a melee weapon
Yes, provided it doesn't already count as a weapon prior to the attack
The act of attacking with it and causing damage is what causes ita non-weapon object to fit the criteria for a weapon and turns a non-weapon object into a weapon. After that attack is over it reverts to being ana non-weapon object.
The corrolary to this is if you are using a proper weapon for a purpose that it was not designed. It remains a weapon for the duration, but while the attack is occuring it is an Improvised Weapon.
PHB pages 147 - 148:
An improvised weapon includes any object you can wield
...
If a character uses a ranged weapon to make a melee attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage.