Force, vigor, or energy; the driving power or vitality behind an action or person.
若い社員には勢いがある。
Young employees have vigor/energy.
彼は勢いよく階段を上った。
He climbed the stairs with great vigor.
スポーツチームに勢いがついてきた。
The sports team is gaining momentum and vigor.
Influence, authority, or power; the effect or strength that something or someone commands.
あの政党は最近勢いがある。
That political party has been gaining influence lately.
会社の勢いに押されて契約した。
I was pushed into signing the contract by the company's strong presence/drive.
新製品の勢いで売上が伸びた。
Sales increased thanks to the momentum of the new product.
Impetus or momentum; the course of events driven by force or momentum (as in 'by/with the momentum of...').
勢いでその車を買ってしまった。
I ended up buying the car on impulse/momentum.
彼の一言で話が勢いづいた。
His remark gave the discussion momentum.
勢いに任せて演説を続けた。
He continued the speech carried along by the momentum.
Used adverbially/connexionally to mean 'as a result' or 'necessarily', indicating a natural or inevitable consequence of the preceding fact.
試験をさぼった。勢い、不合格になってしまった。
I skipped the exam. As a result, I failed.
準備不足の勢いで、本番で失敗した。
Due to lack of preparation, I failed during the real performance.