I think the gravity doesn't affect the mass of an object. Only it's weight can be compared
The answer is gravity. I hope this helps.
Two fat black arrows are swimming along together, when they see a single skinny black arrow coming toward them. They are afraid of strangers, and they know that the skinny one must be mean and tough if it's not afraid to travel alone. So they turn to the side and get out of the skinny arrow's way.
