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In 1 Shmuel 17:49 we read

וַיִּשְׁלַח דָּוִד אֶת־יָדוֹ אֶל־הַכֶּלִי וַיִּקַּח מִשָּׁם אֶבֶן וַיְקַלַּע וַיַּךְ אֶת־הַפְּלִשְׁתִּי אֶל־מִצְחוֹ וַתִּטְבַּע הָאֶבֶן בְּמִצְחוֹ וַיִּפֹּל עַל־פָּנָיו אָרְצָה׃ David put his hand into the bag; he took out a stone and slung it. It struck the Philistine in the forehead; the stone sank into his forehead, and he fell face down on the ground.

David was charging toward Goliat who was also advancing towards him. Based on simple physics when Goliat was struck the force of the impact should have impeded his motion and knocked him backwards, yet the pasuk is explicit that Goliat fell forwards onto his face. Do any commentaries explain why the Plishti didn't fall backwards?

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    Seems unlikely that the force of a stone would have knocked over a giant. It sounds like it killed or wounded him and he fell over as a result.
    – shmosel
    Commented Jul 28, 2023 at 2:44
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    Maybe he tripped over due to the stone? Commented Jul 28, 2023 at 3:59
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    @rikitikitembo It seems your main question is about whether this is addressed in scripture, which is answered by user6591. But with regards to your claim that physics suggests he should have fallen backwards ... no. Goliat was not knocked down by the stone ... he was killed by the stone - it "sank into his forehead". He fell to the ground because he was dead, not because the stone transferred momentum to him.
    – Brondahl
    Commented Jul 28, 2023 at 12:12
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    Running (or walking) forwards consists of consistent falling over forwards, and then catching yourself with the other leg. If you're no longer able to catch yourself with the leg (because you're dead) then you fall all the way forwards onto your face, because that's where your momentum is going.
    – Brondahl
    Commented Jul 28, 2023 at 12:12

4 Answers 4

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Purely from a physics perspective, the mass of the stone, even when moving at speed, is not significant enough to reverse Goliath's forward momentum.

There are many movies where people are shown to fly backwards after being shot. Those are entirely the product of what the director thinks looks cool rather than being a reflection of what would actually happen.

If Goliath were standing still, with his feet side-by-side, it is possible that an impact on the forehead would cause a backwards fall. With a stance with one foot forward of the other (and likely leaning forward) a forward fall becomes almost assured. Include walking, rushing, or running forward, and falling forward is completely expected.

Additionally, the energy of the stone was primarily expended in penetrating the skull. Little energy remained to counter the forward momentum anyway.

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    From a physics perspective he also wouldn't have been KOd from a little rock
    – Double AA
    Commented Jul 28, 2023 at 11:56
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    @DoubleAA In the hands of an expert, a heavy sling bullet or stone could reach speeds of up to 100 mph (160 km/h): "The biggest sling stones are very powerful - they could literally take off the top of your head,". Scientific American article Commented Jul 28, 2023 at 13:00
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    Related to physics: if the impact of the stone would have been enough to knock Goliat over backwards, Newton's 3rd law of Motion would dictate that David received an equal amount of force in the opposite direction, most likely throwing him back significantly further than Goliat was.
    – Nzall
    Commented Jul 28, 2023 at 15:01
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    @DoubleAA The smallest rock that David would have tried to use would be larger and heavier than a rifle bullet.
    – user31998
    Commented Jul 29, 2023 at 6:08
  • A note on the last sentence: energy does not counter momentum: momentum does. Momentum would only have been lost (i.e., not transferred to Goliath's head) if matter had been ejected in the direction of the stone's original path. Losing energy penetrating the skull means there's momentum transfer.
    – wizzwizz4
    Commented Jul 30, 2023 at 21:09
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Rashi addresses this. From Sefaria:

And he fell face downward. He should have fallen backwards since he had been struck on his forehead. But, in order that Dovid should not be troubled to walk [the extra distance] and cut off his head, for Dovid now saved twelve amos and two zeres; for his height was six amos and a zeres, Dovid saved [walking] his entire height back and his entire height forward.

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The Midrash in Vayikra Rabbah 10:7 brings four reasons:

  1. שֶׁלֹּא יִצְטָעֵר אוֹתוֹ צַדִּיק שֵׁשׁ אַמּוֹת וָזָרֶת - (As has already been mentioned), so that the tzaddik Dovid would not have to bother himself to walk six amos and a span (Golyas' height) to reach his head.

  2. עַל שׁוּם דָּגוֹן אֱלֹהָיו שֶׁהָיָה נָתוּן עַל לִבּוֹ - Because Golyas' god Dagon which he carried on his heart - i.e. so that he would fall forwards and thereby crush it

  3. הַפֶּה שֶׁאָמַר וְחֵרֵף וְגִדֵּף יִנָּתֵן בֶּעָפָר - So that the mouth which has blasphemed would be filled with earth

  4. וְיִכָּחֲשׁוּ אֹיְבֶיךָ לָךְ וְאַתָּה עַל בָּמוֹתֵימוֹ תִדְרֹךְ - To fulfil the pasuk (Devarim 33:29) - "Your enemies shall come cringing before you, and you shall tread on their backs".

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re " ... Based on simple physics ... "

There is no good reason to think that your simplified physics are correct.

We do not know what the motion or stance of Golith either befor or at the time of impact. We could easily make up scenarios where he fell forwrds, backwards or sideways. The text tells us what happened without comment on why.

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