• 勢いいきおい — momentum / energy
• 姿勢しせい — posture / attitude
• 大勢おおぜい — crowd / many people
• 勢力せいりょく — influence / power
• 優勢ゆうせい — superiority / predominance
• 情勢じょうせい — situation / state of affairs

勢 combines the power radical 力 with phonetic/semantic elements that suggest growth and pressure. It appears in political, physical, and social contexts to describe influence, movement, or momentum. The kun-yomi いきおい is common in everyday speech for “force” or “trend.”

風かぜの 勢いいきおいが 強つよくなってきた。
The wind is gaining strength.

チームは その勢いいきおいのまま 優勝ゆうしょうした。
The team rode that momentum to victory.