1) HITTING A PERSON TO STOP HIM FROM SINNING
OPINIONS: The Gemara teaches that if an Eved Ivri who was Nirtza refuses to leave his master's home upon the arrival of the Yovel year, and the master hits and wounds him in order to make him leave, the master is exempt from liability. The Gemara initially assumes that the master is exempt because he is allowed to take the law into his own hands and make someone leave his house even when he would otherwise incur no loss (since the Eved, if he stays in the master's home, will have to pay the master for any benefit he receives from the master after his term of servitude has ended).
The Gemara concludes that this Halachah is unrelated to taking the law into one's own hands. Rather, during the Eved's term of servitude, the master gave the Eved a Shifchah Kena'anis, and the Eved now wants to remain with her. It is forbidden mid'Oraisa for a freed Eved to live with a Shifchah Kena'anis. The master, therefore, is allowed to hit him in order to chase him away to prevent him from transgressing an Isur d'Oraisa of living with a Shifchah Kena'anis (RASHI DH Isura).
Does this mean that the Halachah permits one to stop any Jew from doing an Aveirah even if it involves physically harming the Jew? If such conduct is permitted, how does the Halachah prevent an unscrupulous Jew from indiscriminately hitting another Jew, claiming that he saw him transgress an Isur?
(a) The ROSH (3:13) concludes, based on this Sugya, that one is permitted to hit another Jew in order to prevent him from transgressing an Isur. Therefore, if one sees a Jew chasing another Jew to hit him, he may use physical force in order to stop the perpetrator. The ruling of the Rosh is cited by the REMA (CM 421:13).
The TERUMAS HA'DESHEN (#414) presents a similar ruling and permits a Jew to hit his or her spouse in order to prevent the spouse from cursing his or her parents. The REMA cites this ruling as a "Yesh Omrim" (EH 154:3).
This seems to be the opinion of the SHE'ILTOS (#27) who writes that if one sees someone who is not acting properly, he is permitted "to hate him, to curse him, and to do physical acts to him." It seems clear from the She'iltos that not only is one permitted to hit a person who is doing an Aveirah now in order to stop him, one is also permitted to hit him in order to prevent him from returning to that Aveirah in the future. This is also clear from the words of the Terumas ha'Deshen. This is also implied by the Gemara here, since the Eved merely wants to reunite with the Shifchah, and the Gemara permits the master to hit him in order to prevent him from performing that Aveirah in the future. This also seems to be the intention of RASHI in Erchin (16b, DH Yachol Lo Yakenu).
According to this view, when the Gemara in Kesuvos (86a) states that if a person refuses to build a Sukah or hold a Lulav on Sukos, "they beat him until his soul departs," the Gemara does not refer only to Beis Din. Rather, any person may take such measures in order to stop a person from transgressing a Mitzvah. This is how the NESIVOS HA'MISHPAT (CM 3:1) explains the Gemara. (When the Gemara says "until his soul departs," it does not mean that one may actually kill him, since the person is not doing an Aveirah for which one must die instead of transgressing. It means that one may beat him until he is on the verge of death. However, the Nesivos himself in CHAVOS DA'AS (YD 161:6), cited by RAV SHMUEL ROZOVSKY (Chidushim to Bava Basra 10:2) interprets that Gemara as referring specifically to Beis Din, and that it allows them to beat the person to death.)
However, this is not an absolute sanction to hit anyone who acts improperly. The Terumas ha'Deshen permits only a master (in the case of a slave), or someone who is responsible for someone else's actions, to hit the person under his jurisdiction who is acting improperly (when all other measures have been of no avail).
The MAHARSHAL (Yam Shel Shlomo 3:9) writes that only a person who is "Muchzak b'Kashrus," well-known to be upright and G-d-fearing, is permitted to use physical force to stop another person from sinning. This may be based on the same principle as described above; a person who is known to be upright, to whom others look for guidance, has a responsibility to ensure that the people around him act properly.
Why, then, does the Rosh permit hitting someone who is chasing another person to harm him? The Maharshal explains that when the Aveirah that is being done is Bein Adam l'Chaveiro, anyone is permitted to stop the sinner (for the benefit of the person who is being harmed).
How does the Rosh understand the Gemara's proof that a person may take the law into his own hands from the case of the woman who hurts someone who is trying to harm her husband? The Gemara should answer that there, too, the woman is permitted to hurt the person trying to harm her husband only to stop the perpetrator from transgressing the Isur of hitting another Jew!
CHIDUSHEI RAV SHMUEL ROZOVSKY (Bava Basra 10:5) points out that the Rosh himself divides, into two cases, the Halachah of hitting another Jew to prevent him from causing harm to a third Jew. The first case involves a person who sees someone hitting his father, children, or brother, in which case he is allowed to hit the person if he cannot stop him in any other way, because we rule that it is permitted for one to take the law into his own hands. The Rosh continues and says that, similarly, if one sees one Jew trying to harm another Jew and he has no other way to stop him, he is permitted to hit the perpetrator in order to prevent him from doing an Isur. Why does the Rosh give a different reason to allow one to use force in order to save one's close relative? In that case, too, he should say that one is permitted to hit the perpetrator in order to prevent him from doing an Isur! It must be that the Halachah of preventing one from doing an Isur applies only when one derives no personal gain from hitting the perpetrator. When one derives personal gain, such as when the sinner is trying to hit one's close relative (or is trying to hit the person himself), one is not permitted to hit him in order to prevent him from doing an Isur, since it will not be clear that the person's intentions are pure. (That is, even to prevent an Isur Bein Adam la'Chaveiro, there are limitations as to who is allowed to stop another person by force.)
Rav Shmuel Rozovsky points out that this distinction may be derived from the words of Rashi (DH Isura) who specifies that one is permitted to hit someone to prevent him from doing an Isur since "he is not doing it for his own self (for his own benefit)." This is the intention of the Gemara's question from the case of a woman who hits another man to save her husband. Since she does so to save her close relative (her husband), she is not permitted to do so in order to prevent the other person from doing an Isur.
(b) The KETZOS HA'CHOSHEN (in Meshovev Nesivos 3:1) rules that one is permitted to hit another Jew only if he is involved in an Aveirah at that moment. One is not permitted to physically harm him in order to prevent him from performing an Aveirah in the future, for otherwise the law would not require Mumchin (expert judges) to force a man to agree to give a Get when the circumstances require it (see Gitin 88b, and Tosfos to Sanhedrin 2b). Similarly, the Gemara in Kesuvos (86a) permits only Beis Din to beat a person to force him to agree to fulfill a Mitzvas Aseh, since the person is not presently involved in the Isur that they are trying to prevent him from doing. (According to the Rishonim mentioned above, forcing a person to give a Get is different and requires Mumchin because a Get requires "Ratzon" -- he must give it willingly and not just because he is forced. Alternatively, when a person is doing what is clearly an Aveirah, anyone may stop him with physical force, but in a situation where Beis Din is required to determine what the person must do, only the law-enforcement agents (Shotrim) may use physical force to compel someone to do what the Beis Din of Mumchin requires him to do. See CHASAM SOFER, CM 177, DH Gimel.)
(c) When the RAMBAM (Hilchos Avadim 3:5) cites the Gemara, he does not write that the master is permitted to hit the Eved to prevent him from transgressing the Isur, but rather if he hits the Eved he is exempt from damages. The NETZIV (in HA'EMEK SHE'EILAH 27:6) infers from this that the Rambam does not permit one to hit another Jew, l'Chatchilah, even to prevent him from transgressing an Isur. The SHA'AR HA'TZIYUN (OC 608:8) makes a similar inference from the Rambam in Hilchos De'os (6:8), where he discusses the Mitzvah of Tochechah (rebuke) and writes that one may rebuke a known sinner and disgrace him, but he does not write that one may hit him (as the She'iltos writes). In fact, in Hilchos De'os (6:5) the Rambam cites the Gemara from which Rashi learns that one is permitted to hit someone as a form of rebuke, but he explains the Gemara differently. According to the Rambam, the Gemara does not refer to rebuke at all. Rather, it means that if a person hits his friend out of hatred he does not transgress the Isur of "Lo Sisna Es Achicha bi'Lvavecha" -- "Do not hate your brother in your heart" (Vayikra 19:17), since he is expressing his feelings of hatred and not hiding them in his heart.
How does the Rambam understand the Gemara in Kesuvos which teaches that one may use physical force to make a person build a Sukah or hold a Lulav? In fact, the Rambam himself quotes this Gemara in Hilchos Chametz u'Matzah (6:12). Apparently, the Rambam understands that the Gemara's ruling applies only to Beis Din which, as its role as the overseer of the conduct of the people, has a greater responsibility to ensure that Jews follow the Mitzvos properly. (See the SHA'AR HA'TZIYUN, ibid., in the name of the RAMBAN to Devarim 27:26.) (The Rambam does not necessarily mean that Beis Din is different because the judges are Mumchin; even a Beis Din of judges who are not Mumchin may be permitted to hit a person under such circumstances.)

28b----------------------------------------28b

2) DAMAGE CAUSED BY PERSONAL OBJECTS LEFT IN RESHUS HA'RABIM
QUESTION: Rav teaches that when a person leaves his objects in Reshus ha'Rabim and does not make them Hefker, he is liable even for damage which his objects cause to Kelim (unlike a normal Bor). Rav explains that since the owner was not Mafkir the obstacle, it has the status and liability of a Shor and not of a Bor.
Rav's ruling that an object left as an obstacle in Reshus ha'Rabim has the status of a Shor is difficult to understand. Rav apparently compares an obstacle in Reshus ha'Rabim to Keren, and not to Shen and Regel (since one is not liable for Shen and Regel in Reshus ha'Rabim). However, if Rav compares this obstacle to Keren, the owner should pay only Chatzi Nezek, as one pays for Keren!
Moreover, how can Rav compare damage caused by an obstacle, which is inanimate, to damage caused by an animal? A person is more responsible for damage caused by his animal since an animal has "Ru'ach Chaim" (Mishnah 2a), and is "Darcho Leilech ul'Hazik" (end of 9b), and an animal which damages with Keren has intent to damage -- "Kavanaso l'Hazik" (2b, 5b)! An inanimate object has none of these characteristics. (TOSFOS 3b, DH mi'Shoro)
ANSWERS:
(a) TOSFOS (3b) explains that Rav indeed learns that one is liable for such a form of Bor from the Halachah of Keren. With regard to why one pays Nezek Shalem -- and not Chatzi Nezek as for Keren (Tam) -- for damage caused by his object in Reshus ha'Rabim that he was not Mafkir, Tosfos explains that there is no logical distinction between Tam and Mu'ad for inanimate objects. Therefore, even though one pays Chatzi Nezek for the first damages caused by an animal, and thereafter one pays Nezek Shalem, for damages caused by an inanimate object one pays Nezek Shalem for all damages, even the first. (The RA'AVAD and RASHBA here present a similar argument. They explain that these objects are comparable to a Keren that is a Mu'ad since they are "Mu'adim l'Hazik" in that they are in a position to do damage from the start.)
How can the liability for damage caused by an inanimate object be derived from Shor, a more stringent form of liability? Tosfos explains that Rav derives the liability for damage caused by these objects from a Mah ha'Tzad between Shor and Bor, similar to the Mah ha'Tzad mentioned in the Gemara earlier (6a). However, Tosfos asks that if Rav derives from a Mah ha'Tzad that one pays for damages of such objects, he should attribute the leniencies of both sides of the Mah ha'Tzad to this object, and the owner should be exempt from damages done to Kelim (like Bor), since the Halachah that is derived cannot be more stringent than either of the Halachos from which it is derived! Tosfos does not answer this question.
The MAHARI KATZ and RABEINU YECHIEL cited by the Shitah Mekubetzes explain that the primary source for the Halachah that one is liable for one's obstacles left in Reshus ha'Rabim, according to Rav, is Shor, and not Bor, and therefore it has all of the stringencies of Shor.
TOSFOS RABEINU PERETZ and others cited by the Shitah Mekubetzes explain this in more depth. Rav does not learn a Mah ha'Tzad from Shor and Bor equally. Rather, Rav learns this Halachah through a Binyan Av from Shor. The only reason not to learn a Binyan Av is the question mentioned earlier (that one's object is not "Darcho Leilech ul'Hazik" and is not "Kavanaso l'Hazik"). In order to refute those Pirchos, Rav cites the Halachah of Bor, for which one is liable even though Bor, too, lacks those qualities. Once the Pircha has been refuted, there no longer is a basis to ask that perhaps Bor is more stringent because it has qualities that make the owner more responsible, since it has no such qualities. Accordingly, Bor is used simply to refute a Pircha on the Binyan Av from Shor, and it is used as an active partner in the derivation (through a Mah ha'Tzad) of the Halachah of one's obstacles left in Reshus ha'Rabim. (This is called a derivation learned through a "Palga d'Dina"; see MAHARSHA to Tosfos 6a, DH Tomar. See also BRISKER RAV to Hilchos Nizkei Mamon, and SHITAH MEKUBETZES in the name of the GILYON, for a different approach.)
(b) TOSFOS RABEINU PERETZ (3b) and the YAM SHEL SHLOMO (#8) explain that Rav learns that one is liable for damages caused to Kelim by such a Bor through a Mah ha'Tzad from Shor and Esh, and not from Shor and Bor. Since Bor is not used to derive the liability for damage caused by these obstacles, there is no reason to exempt the owner from damage they cause to Kelim. The Shitah Mekubetzes in the name of Mahari Katz points out that even according to this approach, one should be exempt for damages caused by such a Bor to items that are Tamun, just as one is exempt for such damages when caused by Esh.
This explains why Rav obligates one to pay Nezek Shalem for such damages, and why no Pircha may be asked from the fact that a Shor is a living being and has intent to damage. However, another Pircha may be asked: both Shor and Esh are "Darchan Leilech ul'Hazik" -- it is their normal manner to go and cause damage, while the objects to which Rav refers are inanimate and do not move (as the Gemara on 9b indeed considers this a leniency). Why, then, is the liability for damage caused by these objects derived from Shor and Esh?
Perhaps Rav learns like the Ra'avad (9b) who explains the Gemara there differently (i.e. the Gemara does not refer to Keren, but to Regel, and the leniency is that one is exempt for damages of "Dash b'Niro," and not that its nature is different from that of Bor). Alternatively, he may learn like the RASHBA (10a, DH v'Lichechah), who explains that the words "Darcho Leilech ul'Hazik" actually refer to the fact that a Shor has intent to do damage and is a live animal (as the Mishnah says). Accordingly, this Pircha cannot be asked on the Mah ha'Tzad from Shor and Esh.
(c) The RA'AVAD and RASHBA here explain that since the person left his objects in Reshus ha'Rabim knowingly, the damage that they cause is considered done with intent, and thus the owner's liability is as stringent as Shor.
This explains only why no Pircha may be asked from the fact that Shor is "Kavanaso l'Hazik." Why can no Pircha be asked from the fact that a Shor has "Ru'ach Chaim" and is "Darcho Leilech ul'Hazik"? Perhaps these Rishonim are consistent with their own opinions as expressed earlier (9b), where they do not explain "Darcho Leilech ul'Hazik" as being a separate stringency (as mentioned in (b) above). "Ru'ach Chaim" also might not be a Pircha; since it is considered as though these objects cause damage with intent, it is just as stringent as damage caused by a living animal.
(d) The RA'AVAD adds that perhaps the negligence of the person who places the objects in Reshus ha'Rabim is a strong enough factor to make him liable to pay Nezek Shalem, even in Reshus ha'Rabim.
The Ra'avad apparently understands that the liability for these obstacles is derived from a Shor which causes damage with Regel, which is "Derech Hilucho," since these obstacles also cause damage by doing what they normally do (remaining still). Nevertheless, in contrast to Regel, one is not exempt for damage caused by these obstacles in Reshus ha'Rabim, since the negligence of the owner is involved in the damage more than it is involved when one's animal causes damage. Therefore, it is not reasonable to give it the leniencies that apply when a person's animal causes damage.
(e) The SHITAH MEKUBETZES cites TOSFOS TOSFEI HA'ROSH here who explains that the liability for damage caused by objects placed in Reshus ha'Rabim is derived not from Shor but from a Bor which is nine Tefachim deep. The Yerushalmi (5:6; see Insights to 4b) explains that one is exempt from liability for the death caused to a person and for damage caused to Kelim only when it one's ten-Tefach-deep Bor which caused the damage. For one's nine-Tefach Bor, however, for which one is liable for damages done to a person (like one is liable for damages done to an animal), one is also liable for damage done to Kelim. These obstacles are comparable to a nine-Tefach-deep Bor, and that is why one is liable for damage they cause to Kelim. (The RASHBA on 53b, however, points out that the Bavli here and on 35a does not seem to agree with the Yerushalmi.)