Q. Can my children use a tire swing on Shabbos? The rope is attached to a metal hook that protrudes from a tree. When a child swings on the swing the tree branch it does sway but only very slightly.
A. Shulchan Aruch (O.H. 336: 13) rules that the prohibition of using a tree on Shabbos includes also objects attached directly to it, commonly called "Tz'dadim" or sides. However, one may use other objects attached to the first ones or sides of sides. The example given is the prohibited use of a ladder that leans directly on a tree but permitted when resting on a beam or post attached to the trunk. Shemiras Shabbos Kehichoso (26: 17) adds that the use of a swing that hangs from a beam or bar attached to a tree is permitted, as it constitutes sides of sides. The hook you describe is not part of the tree and it would also be considered a side while the tire swing would be sides of sides. The swing cannot be attached or removed from the hook on Shabbos since that would be using directly the hook (ibid.). Mishna Berura (ibid. 63) and Shemiras Shabbos (ibid.) mention that it is only permissible if the tree does not sway.
Horav Shlomo Miller's Shlit"a opinion is that if the movement can be readily noticed without special scrutiny or dedicated observation, the swing should not be used.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a
Important Numbers
