• The force that the surface are being pushed together.
Explanation:
Friction can be said to be the general term used to describe the force that one surface exert on another when they rub against each other.
When the surface is partially not smooth or totally rough, a greater force is been needed to move one surface past the other.
The weight of the can also significantly be a key factor. This is where the need to friction reduction come to play a role; and need for friction reducers like lubricant, rollers etc helps.
Catalyzed reactions are typically used to accelerate the rate by which a specific chemistry proceeds. Essentially, the action of the catalyst is to provide an alternative, lower energy pathway for the reaction.
Laws explain what occurs, not why it occurs. This is a law because it states that a limp stalk of celery will become crisp if put into ice water. There is no explanation why or how it occurs, only that it will happen.