Gemara

(R. Ami): If Reuven wounded Shimon and the compensation would be less than a Perutah, he is lashed instead;

Sanhedrin 58b (Reish Lakish): One who raises his hand to strike someone is called a Rasha, even if he did not hit him - "va'Yomer la'Rasha Lamah Sakeh Re'echa";

It does not say 'why did you hit him?', rather, 'why will you?' He is called a Rasha even without hitting him.

(Ze'iri): He is called a Chotei. It says "v'Im Lo Lokachti b'Chazakah" (threatening to take), and it says "va'Tehi Chatas ha'Ne'arim Gedolah Me'od."

Bava Kama 28a (Beraisa): If a Nirtza (a slave whose ear was pierced) did not want to leave (in Yovel, the 50th year, when he goes free) and his master was trying to force him to leave, and injured him, the master is exempt.

(Rav Nachman Bar Yitzchak): The case is, the master designated a Shifchah Kena'anis for his slave. Now he is free, and he is forbidden to her, so the master may act to stop him from sinning.

91a - Question: Must we evaluate for damage (whether the damaging object was apt to cause the damage)? Perhaps we evaluate only for killing, for not everything can kill, but anything can (do some) damage. Or, also for damage, we must evaluate the object.

Answer (Beraisa - Shimon ha'Teimani): "When a man will hit his fellow man with a rock or his fist" - just like the judges and witnesses can see his fist (and gauge whether it is apt to cause the damage that resulted), they must see anything used to hit;

This teaches that we evaluate for damages.

Rishonim

Rif and Rosh (Bava Kama 32a and 8:10): There was no question that we estimate the victim, to see what is able to make him bedridden. The question was about estimating the Keli. We answered that the Keli must be like a fist, which the congregation and witnesses can see.

Rambam (Hilchos Chovel 1:18): There is estimation for damage just like for murder. If Ploni hit Reuven with a small stone or chip of wood that should not cause such damage (as resulted), he is exempt. "B'Even Oh b'Egrof" teaches that the weapon must be capable of damaging. However, he is liable for embarrassment. Even one who spits on another is liable for Boshes. Therefore, the witnesses must know with what he hit him. They bring it to Beis Din, and they assess it and judge it. If it was lost, and Ploni says that it was not capable of damaging and this is like Ones, and Reuven says that it was capable of damaging, Reuven swears and receives, like we will explain.

Support (Ra'avad): When witnesses do not know whether Ploni wounded Reuven or Reuven wounded himself, Reuven swears and receives, for it is abnormal to wound oneself. We follow Chazakos. Here, witnesses saw that he damaged, and a small stone does not normally do such damage, so all the more so Reuven is believed through an oath.

Rambam (19): Even a small piece of iron can kill. One need not say that it can damage!

Rambam (5:1): One may not wound himself or his fellow man. A Lav forbids hitting a Kosher Yisrael, child or adult, man or woman, in the way of fighting - "Lo Yosif Lehakoso." The Torah warns not to strike a sinner more than he deserves, and all the more so not to hit a Tzadik!

Rambam (2): If one lifts his hand to strike someone, even if he did not hit him, he is called a Rasha.

Hagahos Maimoniyos (5:1 and Shiltei ha'Giborim Makos 2b): Ra'avan says that one who lifts his hand to strike someone is called a Rasha. He is Pasul (for testimony) and one who claims against him swears and collects, until he repents and accepts the judgment, even if someone other than the victim claimed against him.

Rambam (3): If Reuven wounded Shimon and the compensation would be less than a Perutah, he is lashed.

Terumas ha'Deshen (218): If one's wife curses and disgraces her parents, and he verbally rebuked her several times but it did not help, he may hit her. Even though the Mordechai brought in the name of Rabbeinu Simchah that one who hits his wife transgresses the Lav of Pen Yosif, and he is very stringent about this, it is permitted to stop one from Isur. A proof is from Bava Kama 28a. If a master insists that a Nirtza leave, and the slave refuses, if he hit him, he is exempt. We established this to be when his master designated a Shifchah for him, and he hit him to separate him from Isur. Rashi explains that he may hit him. One may not take the law into his own hands for himself, only for Shamayim. This shows that anyone under one's control, if he sees him sinning, he may hit and afflict him to stop him. He need not bring him to Beis Din for them to separate him through the strength of their decrees. If we would not say so, why may a master hit the Nirtza? He should take him to Beis Din!

R. Yerucham (Meisharim 31:2 92:4): There is estimation for damage just like for murder. We assess the strength of the one who hit and the victim, the limb he struck, whether he hit him from close or far away, and the rock or stick he used to hit him, to determine whether it was proper to do this damage. The witnesses must know with what he hit him. If it should not have done the damage, he is exempt, for we require something capable of damaging. The estimation is in the eyes of the witnesses, not in Beis Din, for it is not needed.

Poskim

Shulchan Aruch (CM 420:1): One may not hit his fellow man. If he did, he transgressed a Lav "Pen Yosif." The Torah was insistent not to strike a Rasha more than he deserves, and all the more so not to hit a Tzadik! If one lifts his hand to strike someone, he is called a Rasha.

Rema: Some say that there is an old Cherem not to hit someone, and one who did so must be permitted before he can join to a Minyan of 10. Once he agrees to a judgment, we permit him, even if the victim is not appeased.

Darchei Moshe (1): Mahari Veil (p.172) says that he does not join for any matter of Kedushah until he accepts whatever Beis Din will want.

SMA (4): The Rema connotes that early Chachamim already made the Cherem, and nowadays we need not excommunicate him. I say that the same applies when Acharonim say that the congregations made a Cherem, or the Cherem of R. Gershom. One who transgresses is automatically in Cherem due to the decree. The Rema connotes that standard permission of Cherem in front of three helps. This is unlike a Chacham who said that whenever Acharonim say that the congregations made a Cherem, we need 100 people from three congregations in three lands to permit it. We find this only regarding one who wants to marry another wife while still married, for a need. This is only for a big Heter like this, but not for other Cheremos of congregations or of R. Gershom (i.e. not to divorce a woman against her will).

Chasam Sofer (CM 182, cited in Pischei Teshuvah 1): A Rasha (Ploni) punched a Ben Torah and wounded him. Ploni is a mourner over a parent, and is the Shali'ach Tzibur every day. Even though he is in Cherem, he is not like a Kusi. We answer Amen to his Berachos, for he blesses Hash-m. It is a fine not to join him for a Minyan. If he makes himself Shali'ach Tzibur by force, we do not answer. This is to chatise him until he repents. He would not care if `individuals do not answer. They will miss answering, and he will not repent! Hash-m will weed out the thorns from His vineyard. Nowadays, if we would be exacting about everyone Chayav Niduy due to enactments, we could not deal with most of our generation.

Shulchan Aruch (2): If Reuven wounded Shimon and the compensation would be less than a Perutah, he is lashed, since there is no monetary Chiyuv.

SMA (5): If there is a monetary Chiyuv, he pays and he is not lashed, even if he was warned.

Shulchan Aruch (OC 55:11): If one transgressed a decree of the Tzibur or an Aveirah, if he was not excommunicated he joins for a Minyan.

Beis Yosef (DH Kasuv): Even a Yisrael who sinned is a Yisrael (Sanhedrin 44a).