To return to this:
I thought only when you say the name of
Eru they get that "physical force" as you said. Is it not true then?
How exactly this applied to the
Elves, I don't know, but we do see the effects of oaths throughout
Tolkien's works. In addition to the Deadmen, it's seen in "Cirion and
Eorl" in Unfinished
Tales; Cirion's oath does cite Eru ('
I Eru i '; 'of
the One who'), but Eorl's does not:
[. . .] 'and therefore I vow in my own name and on behalf of the Eothed of the North [. . .] Therefore this vow shall descend to my heirs, all such as may come after me in our new land, and let them keep it in faith unbroken, lest the Shadow fall upon them and they become accursed'.
As far as I'm aware, these people had never yet heard of Eru (though I could be mistaken, and would welcome correction). Nevertheless, they considered the oath unbreakable, and in fact the wording could imply that the Shadow
would "fall upon" anyone who broke it. In any event, both
peoples held the oaths "sacred" and kept them down through the
Ages, including the Fourth. (I can't resist observing here how the nonsense inserted by PJ&Co. for "dramatic tension" makes a particularly annoying mockery of this, but that's by the way.)
Nor do I know if the Deadmen had heard of Eru, though they had worshipped
Sauron. The text doesn't mention him in relation to their oath, but as I said above, the effects of breaking it were definitely "physical", among other things.
We tend to forget the power of oaths and words to pre-literate people, before the age of written contracts and squabbling litigation, so amusingly satirized in the episode of
Bilbo's will, and "Grubb, Grubb, and Burrowes" in
The Hobbit, where "the legal bother, indeed, lasted for years". As said previously, a reading of the sagas will impress this.
Lastly, there's another example in LOTR:
'
Smeagol,' said
Gollum suddenly and clearly, opening his eyes wide and staring at Frodo with a strange light. 'Smeagol will swear on the
Precious.'
Frodo drew himself up, and again
Sam was startled by his words and his stern voice. 'On the Precious? How dare you?' he said.
Nonetheless, he does dare. One can imagine, say, a Norseman from the 9th century sitting up, on having this read to him.
There have been decades of discussion and argument about what exactly caused Gollum's fall into the fire, one of them being direct intervention by Eru. We've seen it here recently, and I don't doubt someone somewhere has posted a picture of a giant ghostly finger flicking him over the edge. But Tolkien, as usual, is far more subtle and ambiguous for such a simple, one-level explanation.
I can easily picture our imaginary Norseman listening on, until we come to the passage where Smeagol/Gollum argues himself into breaking his oath.
"
There", he'd say; "
There. He has doomed himself".
