"Momentum" in Japanese can be translated as 「勢い」 (ikioi) or 「運動量」 (undōryō).

- 勢い (ikioi): This is the more general term for momentum, often used metaphorically to describe a forward-moving force or enthusiasm.

- 運動量 (undōryō): This is the more technical/scientific term that directly translates to "momentum" in physics.

If you're referring to "Japanese momentum" in a specific context, such as economic growth or progress in a particular field, it would typically be expressed using 勢い (ikioi) or 力強さ (chikara-zuyosa, meaning strength/force/power).

For example:

- 日本の経済の勢い (Nihon no keizai no ikioi): The momentum of Japan's economy

- 日本の技術力の向上の勢い (Nihon no gijutsuryoku no jōshō no ikioi): The momentum of improvement in Japan's technological capabilities

Let me know if you need more specific information or a different context!