“〜hazumi de・no ikioi de”: On the Spur of / Carried by the Momentum [JLPT N1]

Content by NTJ Kuma · Nihongo to Japan editorial team · Updated July 27, 2026

「転んだはずみで足をひねった」「酒の勢いで告白した」 — swept along by the moment, not by plan.

「話のはずみで、つい言ってしまった」 — not planned, but carried by the momentum of the moment

To depict “not deliberate, but done on a momentary impulse,” 「話のはずみで、つい言わなくてもいいことを口にした」 (carried along by the conversation, I blurted out something I needn't have) — 「〜はずみで・〜の勢いで」 = on the momentum of, in the heat of the moment (not deliberate), used to express “under some momentary impetus, inertia, or high spirits, one (involuntarily) did something.” This guide covers attachment, the common verb た-form, the reading trap, and separating it from 「拍子に.」

Core: not planned, but carried by the momentum of the moment

「(noun の／verb た-form) ＋ はずみで／noun の ＋ 勢いで」 means “under the momentary impulse, inertia, or high spirits of that instant, one (involuntarily) did something”:

• 電車が急停車したはずみで、立っていた乗客同士がぶつかった (the instant the train braked hard, standing passengers collided).
• その場の勢いで、契約書にサインしてしまった (in the heat of the moment, I ended up signing the contract).
• 転んだはずみで、足首をひねった (the instant I fell, I twisted my ankle).

Feel: the core is “not deliberate, but carried by that instant's motion inertia or emotional high spirits.” 「はずみで」 leans “that instant's inertia／going with the flow” (話のはずみで = carried by the conversation; 転んだはずみで = the inertia of falling); 「の勢いで」 leans “on the impetus of (a burst of) high spirits／impulse” (その場の勢いで = the momentum of the moment; 酒の勢いで = on the strength of drink). It corresponds to “on a momentary impulse, on the momentum of, blurting／doing involuntarily, on the strength of ～'s impetus.”

Forms & attachment

Example sentences

• 勢いあまって、余計なことまで言ってしまった。(Carried past myself, I said even unnecessary things.)
• 席を立ったはずみで、コップを倒してしまった。(The instant I stood up, I knocked over the cup.)
• その場の勢いで引き受けたが、後で後悔した。(I took it on in the heat of the moment, then regretted it.)
• 何かのはずみで、昔のことを思い出した。(By some chance impulse, I recalled the past.)
• ノリと勢いで始めた企画が、意外とうまくいった。(A project started on a whim and momentum went surprisingly well.)

⚠️ Common mistakes to avoid

① The core is ‘under the momentary impetus／inertia／high spirits (not deliberate), one did something.' 「〜はずみで・〜の勢いで」's core is “not through deliberation, but carried (involuntarily) by ‘that instant's motion inertia or emotional high spirits.'” It stresses “that moment, that impetus” — not a rational plan, but a going-with-the-flow／inertia: 話のはずみで、つい本音を漏らした (carried by the conversation, I let slip my true feelings), その場の勢いで、みんなの前で告白した (in the heat of the moment, I confessed before everyone). It corresponds to “on a momentary impulse, on the momentum of, blurting involuntarily.” Test: to say “(not planned,) carried by the moment's inertia／high spirits, one did something” → はずみで／の勢いで.

② Attach a noun の／verb た-form (not the dictionary form); 「勢い」 reads 「いきおい.」 Two error-prone points. Attachment: 「はずみで」 takes a noun ＋ の (話のはずみで) or a verb た-form (転んだはずみで, ぶつかったはずみで), not the dictionary form (✗ 約束するはずみで → ✓ 約束したはずみで). 「勢いで」 takes a noun ＋ の (その場の勢いで). Reading: 「勢い」 reads the kun-reading 「いきおい,」 not the on-reading 「せい」 — don't reflexively read 「勢」 as 「せい」 (「せい」 is another word's reading). Mnemonic: noun の／verb た-form ＋ はずみで; 勢い = いきおい.

③ Near-synonym with 「拍子（ひょうし）に」 (often tested together at N1); division of labor with 「ついでに／うっかり.」 「〜はずみで」 vs 「〜拍子に」: 「(verb た-form／noun の) ＋ 拍子に」 also expresses “the instant of ～, the moment ～ (going with the flow, unexpectedly),” close to 「はずみで,」 mostly interchangeable: 転んだ拍子に足をひねった ≒ 転んだはずみで足をひねった (the instant I fell, I twisted my foot) — both “at a motion's instant／inertia, (unexpectedly) something happened.” And separate the other two: 「ついでに」 = in passing (do another thing incidentally on an occasion, see 〈ついでに〉): 買い物のついでに (in passing) — “one trip, two things,” not “carried by momentum.” 「うっかり」 = carelessly, by oversight (plain carelessness): うっかり忘れた (carelessly forgot) — “carelessness,” without “the moment's high spirits.” Difference: 「はずみで／勢いで」 = carried by the moment's inertia／high spirits (not deliberate); 「拍子に」 ≒ はずみで (an instant, going with the flow); 「ついでに」 = in passing; 「うっかり」 = carelessly. Test: “carried by the moment's impetus／inertia (did)” → はずみで／勢いで (≒ 拍子に); “in passing” → ついでに; “carelessly” → うっかり.

🔄 Compare with similar forms: はずみで／の勢いで／拍子に／ついでに

💡 The one thing to remember

「〜はずみで・〜の勢いで」 = on the momentum of, in the heat of the moment (not deliberate) — three points: ① the core is “carried (involuntarily) by the moment's impetus／inertia／high spirits, one did something” — not planned (話のはずみで／その場の勢いで); ② attach a noun の／verb た-form (✗ dictionary form); 「勢い」 reads 「いきおい」 (don't read it せい); ③ near-synonym with 「拍子に」 (mostly interchangeable), with a division of labor from 「ついでに (in passing)／うっかり (careless).」 This pattern vividly captures that “went with the momentum in the moment, only later thinking ‘why did I…'” feeling — blurting something out, agreeing in the heat of the moment, twisting an ankle from a fall's inertia. To depict “on a momentary impulse, carried by the moment,” 「〜はずみで／の勢いで」 is spot-on.

❓ FAQ

Q: What's the difference between 「はずみで」 and 「勢いで」?

Close in meaning (both “carried by the moment's impetus”), the difference is the emphasis on ‘inertia' vs ‘high spirits.' 「〜はずみで」 = that instant's inertia, going with the flow (leaning ‘motion inertia, following a motion／conversation'): it leans “the momentary inertia of some motion, or following the flow of a conversation,” often with an “unexpected, involuntary” flavor, mostly with (physical) motions or a conversation's flow: 転んだはずみで、手をついた (the instant I fell, I put a hand down — the physical inertia of falling), 話のはずみで、余計なことを言った (carried by the conversation, I said unnecessary things — the conversation's flow), 電車が揺れたはずみで、よろけた (the instant the train swayed, I staggered). It's mostly “(some motion's) momentary inertia bringing out another thing.” 「〜の勢いで」 = on the impetus of (a burst of) high spirits, impulse (leaning ‘emotional／atmospheric impetus'): it leans “on the impetus of a burst of emotion, atmosphere, impulse,” often with an “in one go, on impulse” flavor, mostly with emotion, atmosphere, drink: 酒の勢いで、上司に本音をぶつけた (on the strength of drink, I told my boss my true feelings — the impetus of drink), その場の勢いで、みんなで旅行を決めた (in the heat of the moment, we all decided to travel — the atmosphere's impetus), 若さの勢いで、会社を辞めて起業した (on youthful impetus, I quit and started a business). It's mostly “on the impetus of a burst of emotion／atmosphere, doing (some decision/action).” Difference: 「はずみで」 leans “(a motion/conversation's) momentary inertia, going with the flow”; 「勢いで」 leans “(an emotion/atmosphere's) burst of impetus, high spirits.” Test: to say “some motion's momentary inertia, following the conversation (unexpectedly) ～” → はずみで; to say “on the impetus of a burst of emotion/atmosphere (doing some decision/action)” → の勢いで. E.g. 「ぶつかったはずみで、荷物を落とした (the instant I bumped, I dropped my bag — motion inertia)」 uses はずみで; 「ノリと勢いで参加を決めた (decided to join on a whim and momentum — atmospheric impetus)」 uses 勢いで.

Q: Do 「転んだはずみで」 and 「転んだ拍子に」 mean the same?

Almost the same (both “the instant of falling, going with the flow ～”), mostly interchangeable, the difference very subtle. Both express “at some motion's instant, following that motion's inertia, (unexpectedly) another thing happened,” near-synonym patterns often tested together at N1. 「〜はずみで」 = the instant of ～, going with the flow (stressing the ‘motion's inertia, impetus' bringing out a result): 転んだはずみで、足首をひねった (the instant I fell, I twisted my ankle — “falling”'s inertia bringing out “twisting”). 「〜拍子に」 = the instant of ～, the moment ～ (stressing that ‘the instant／timing of that motion' another thing happened simultaneously): 転んだ拍子に、足首をひねった (at the instant of falling, I twisted my ankle — at “falling”'s instant, simultaneously “twisting”). The two mean almost the same, both “the moment ～ (going with the flow, unexpectedly) ～,” and in practice are mostly interchangeable (転んだはずみで／拍子に, 立った拍子に／はずみで). If pressed for a subtle difference: 「はずみで」 stresses more the motion's ‘inertia, impetus' (that force carrying the doing/happening); 「拍子に」 stresses more ‘the instant, timing' of the motion happening (at that time point, simultaneously). But this difference is very subtle, and in exams and practice the two are treated as synonyms, interchangeable. Test: to express “at some motion's instant, going with the flow (unexpectedly) another thing happened,” 「はずみで」 or 「拍子に」 both work (same attachment: verb た-form／noun の ＋ はずみで／拍子に). Just remember these two are “a set of interchangeable near-synonyms.”

🖊️ Practice Questions (5)

Q1. 「___、つい余計なことを言ってしまった。」(Getting carried away in the flow of conversation)

(A) 話のはずみで　(B) 話のおかげで　(C) 話のせいで(nuance differs)　(D) 話のために

Q2. 「酒の勢いで」の意味は?

(A) Emboldened by drink

(B) After quitting drinking

(C) Didn't drink

(D) Hates alcohol

Q3. 「転んだはずみで足をひねった」の意味は?

(A) Twisted the ankle in the moment of falling

(B) Twisted it on purpose

(C) Didn't fall

(D) Twisted it before falling

Q4. 「はずみで」の語感は?

(A) On the spur/momentum of the moment (not deliberate)

(B) After careful deliberation

(C) Command

(D) Condition

Q5. 接法が正しいのは?

(A) その場のはずみで約束した。　(B) 約束するはずみで。

(C) はずみでを言う。　(D) はずみだで言う。

Answer Explanations

1. (A) 話のはずみで ── Getting carried away in the flow of conversation.

2. (A) ── Emboldened by drink.

3. (A) ── Twisted the ankle in the moment of falling.

4. (A) ── On the momentum of the moment (not deliberate).

5. (A) ── 「その場のはずみで」 is correct.