勢 means “force, momentum”

いきおい · ikioi

Also: forces, energy, military strength

Kun'yomi (Japanese reading)
いきおい, はずみ
On'yomi (Chinese reading)
セイ, ゼイ

JLPT N2 · Taught in grade 5 · 13 strokes

The story

Picture someone planting a seedling (埶) with all their strength (力): growth that keeps pushing upward. 勢い is momentum, 大勢 is a crowd of people, and 勢力 is the power a faction wields in politics.

Used in 勢い (ikioi, momentum/force), 大勢 (oozei, crowd/many people), and 姿勢 (shisei, posture/attitude)

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Readings from KANJIDIC2, © EDRDG, CC BY-SA.