What is the question "ha'Yerutzun ba'Sela Susim"?
Rashi: Will they persist running when they change the order of the world?! Radak ? "Sela" below applies also here. It is not normal for horses to run on rocks!
Malbim: Sela is a hard mountain. It is not proper for anything. One cannot pave a path on it for horses to run on at a time of war, and one cannot plow it with cattle to make a grain field. This is a Mashal for their land, which became like rock and flint. They cannot fight on it in war to be saved from the enemy, and they cannot seed in it to be saved from hunger. It also hints to their heart ? it became like a rock, and cannot stand in the war of the Yetzer ha'Ra, and does not receive seeds of Musar and Da'as.
What is the question "Im Yacharosh ba'Bekarim"?
Rashi: Can one plow in rock with cattle? You intend to change the good path, and you will endure and succeed! Radak ? the verse omitted the subject (one who plows). There are many verses like this, e.g. "Ka'asher Yishbor Es Kli ha'Yotzer" (Yirmeyah 19:11) ? the one who breaks. "Asher Yakriv Mimenu Korban la'Shem" (Vayikra 27:9) ? the one who offers. "Va'Yomer l'Yosef" (Bereishis 48:1) ? the one who told him. If one would plow in rock, this is opposite the way of the world. You do the opposite of the world ? Mishpat and Tzedakah make the world endure. One who makes Mishpat crooked, he destroys settlement of the world, just like Rosh and La'anah (gall and wormwood) kill man. Also above, it says "ha'Hofechim l'La'anah Mishpat" (5:7).
What do we learn from "[Hafachtem l'Rosh Mishpat] u'Fri Tzedakah l'La'anah"?
Radak: The fruit of Tzedakah should be Shalom and good ? "v'Hayah Ma'aseh ha'Tzedakah Shalom" (Yeshayah 32:17), and you make it La'anah!
Malbim: A requisite to win in war is to do Mishpat Bein Adam l'Chavero. It says about Yehoshafat, that when he stationed judges in every city, he defeated his enemies around him (Divrei ha'Yamim II, Perakim 19-20). Sword comes to the world for distorted judgment (Avos 5:8). In place of Mishpat sweetening all bitterness, you reversed it to Rosh (gall). There is pleasant life in society when there is Mishpat Bein Adam l'Chavero ? you reversed it to bitterness. The requisite for seeds to sprout and produce grain of Tzedakah Bein Adam la'Makom. Via this, Hashem oversees and gives rain in its time. You reversed Pri Tzedakah to La'anah. You do the opposite of Tzedakah ? you serve idolatry, "v'Atzar Es ha'Shamayim? v'Lo Yihyeh Matar" (Devarim 11:17), "veha'Adamah Tisa'eh Shemamah" (Yeshayah 6:11).
