The Unseen Servant spell creates
...an invisible, mindless, shapeless force...
...It has AC 10, 1 hit point, and a Strength of 2, and it can't attack. If it drops to 0 hit points, the spell ends...
This shapeless force has a strength score and can interact with objects, including carrying them. It can take damage, though what forms that damage take are left unspecified. Does this mean an Unseen Servant can be physically manipulated and interacted with? Here are a series of related sub-questions:
1) Can an Unseen Servant be picked up, shoved, carried, or pushed aside?
2) Can it be commanded to "ride" or "hang on" to another willing character (such as the party fighter) as a means of allowing it to keep up with a group moving faster than 15ft a round?
2b) If so, does it have weight?
3) Can an Unseen Servant displace water, fog, and other substances?
3b) If so, can it be used to create a small "window" in fog, allowing a character in fog to see through the servant to a space without fog?
3c) If it does displace other materials, could careful observation of this phenomenon allow a character to "spot" an Unseen Servant?