May one lie on a swing or hammock which is attached to a tree on Shabbos?
Background: It is forbidden to make use of a tree, or of items which are being directly supported by a tree or its branches [that are above three Tefach-24 cm-from the ground]. [Such an item is referred to in Halacha as Tzedadin.] However, one may make use of items that are not directly supported by a tree even though they are being supported by an item that is supported by the tree. Thus, if an item is attached to a nail that was hammered into a tree, one may make a use of that item. [Such an item is referred to in Halacha as Tzedei Tzedadin.] However, this is only permitted on condition that it will not inevitably cause the tree or its branches to shake, otherwise it is forbidden even though it is Tzidei Tzedadin. [An item is considered to be directly supported by a tree even if it contains many parts and only its last piece is directly attached, while the remainder of it is supported by the attached part. Thus, it is only considered Tzidei Tzedadin if it is attached to an item that has been fixed onto the tree, such as a nail.]
Directly attached to tree: Accordingly, if a hammock or swing is directly attached to a tree [in an area that is above three Tefach from the ground, which is almost always the case], the hammock is forbidden to be used. This applies even though the hammock or swing contains a hook, chain or rope which supports the hammock material, and only its chain or rope is directly attached to the tree.
Indirectly attached to tree: If the hammock or swing is not directly attached to the tree but is rather attached to a metal pole or nail which has been fixed onto the tree, then its permissibility depends on whether the tree or its branches shake during its use. If using it will cause the tree or its branches to shake in the process, then it is prohibited to be used even though it is indirectly attached. If, however, no part of the tree will shake in the process of using the hammock, then it may be used. [However, some Poskim forbid its use entirely under the claim it is difficult to ascertain whether such action causes the tree to shake. If the hammock is attached to the tree from below three Tefach from the ground, it may be used in all cases.]
Attaching and detaching it to and from the tree: It is forbidden to attach an item to a tree even indirectly, such as to hang it on an item that is being supported by the tree [i.e. a nail]. Thus, the hammock or swing must be set up before Shabbos on the protruding nail for one to be allowed to swing on it on Shabbos. Likewise, it is forbidden to remove an item from a tree even if it is only indirectly attached, such as if it is hanging on an item that is being supported by the tree [i.e. a nail]. Thus, the hammock or swing must be removed before Shabbos from the protruding nail if one does not want it there over Shabbos.
It is forbidden to attach a hammock or swing to a tree, or to a nail that is fixed on a tree, on Shabbos. It is likewise forbidden to be removed. If it was attached before Shabbos, it may only be used on Shabbos if a) It is attached to a nail that is fixed on the tree, and not to the tree itself and b) The tree does not shake in the process of it being used.
 See Admur 336:20; Beir Moshe 6:29; SSH”K 16:16; Piskeiy Teshuvos 336:23
 Admur 336:1 regarding tree; 336:20 regarding Tzedadin; Michaber 336:1 and 13; Shabbos 154b-155a; Eiruvin 32b
 Admur 336:6
 The reason: As by doing so one is making use of the side of the tree which he is using as support. [Admur ibid; Michaber ibid; Rav Ashi in Shabbos ibid]
 The reason: As only the use of the side of the tree was decreed against, while the use of “the sides of the side” was permitted. [Admur ibid; Michaber ibid; Rav Ashi in Shabbos ibid]
 The reason: As the nail is considered the side while the ladder is the side of the side which is allowed to be used. [Admur ibid; Michaber ibid; Rav Ashi in Shabbos ibid]
 Admur ibid; M”A 336:15; Rav Papa 155a; M”B 336:63
The reason: As if one causes it to shake he has made use of the tree itself, which is forbidden. [Admur ibid]
 See M”B 305:67; Piskeiy Teshuvos 336:23; Maor Hashabbos 2:28; Or Letziyon 2:38
 Piskeiy Teshuvos ibid footnote 186
 SSH”K 16:16
 Beir Moshe 6:29
 Admur 336:1 regarding a tree itself; 336:20 regarding removing an item from the basket [Vetzaruch Iyun regarding the allowance of Admur to lean the ladder on the nail which seems to contradict this notion; See Michaber 336:13; M”B 336:60; 62 in name of Achronim; Kaf Hachaim 336:85 that the intent is from before Shabbos]; SSH”K 16:16