@JeremyECrawford 1 more question: Can be the effect of phantasmal force a bag on the target’s head which is moving with the target?
— Radim Havlíček (@Tavicz) August 24, 2015
Yes, assuming the illusory bag can fit in a 10-foot cube. https://t.co/2jVMDGxOE8
— Jeremy Crawford (@JeremyECrawford) August 25, 2015
But can that illusory 2x2x2ft bag move with the target, if the target moves beyond the 10ft area?
The 10ft cube is a constraint on the dimensions of the illusory object: “you create a phantasmal object […] no larger than a 10-foot cube”.
By RAW and JC’s Sage Advice, a suitable illusion (bag on the head, a fire in their pocket, a direwolf snapping at their britches) moves wherever the target goes because the illusion is in their head. It’s not Minor Illusion.
Lava flows and fog clouds are a different question. Silent Image can be moved as an action. Phantasmal Force lacks that wording, frustratingly.