As the player add or removes items increase/decrease their mass. So it's their default mass + the sum of all items in inventory being set as the player's new mass.
Knock objects out of player's inventory
When player runs into another game object, randomly select an item in player's inventory and get it's mass. compare the game object the player ran into against the item in the inventory and drop it to the ground if the item in inventory has less mass.
(if both gameobjects are NPC on NPC or they are Player on NPC, the one that will have their inventory checked for knockout is the person with less mass overall)