勢い
いきおい · ikioi · JLPT N3 · noun
vigor
🚂 勢 = 埶 (cultivate, grow) + 力 (power): power that keeps growing
The kanji 勢 has 力, power, at its base, under 埶, to plant or cultivate. Momentum is power that builds as it goes. For the sound, ikioi works like a yell to keep a push going: I keep going, oi!