1/2mv2

Ok. I understand what you asked. Let me define KE first and then i will answer your question step by step

1) Define KE:

An object of mass m, starting from rest, increases its velocity to v. Then the associated KE (at that particular moment) is given by 0.5*mv^2

2) There are a few possible reasons why the object can move in constant velocity.

2.1) The first possible reaon is that there is no energy loss. After the object reaches velocity v, no more energy is injected. Because of zero energy loss, the object moves at velocity v. the associated KE is still 0.5*mv^2

2.2) the second possible reason is there is energy loss, but at the same time I inject energy to the object. For example, I use my finger to push the rubber on a rough surface at a constant velocity v. since it is constant velocity, the associated KE is also 0.5*mv^2

3) In your question, actually there is no physical difference between the first and the second scenario. The only difference is the way how you describe them.

4) To sum up, you can understand this in the following way:

An object was initally at rest (didn't move). then it accelerates to velocity v. at this particular moment, the associated KE is 0.5*mv^2.

Once the velocity reaches v, it stops acceleration. If there is no energy loss and the net force on this object is zero, the object will move in constant velocity. For anytime onwards, the associated KE is 0.5*mv^2 because the velocity doesnt change. Or you can say because there is no energy loss, the KE can keep its constant level.

**PS: I only confine the discussion into linear motion, as I see you said 向前行. I try not to make things too complicated.

這問題很簡單，一句說話便可答完。

答案是一樣

物體的動能是 (1/2)mv2，無論什麼情況只要速率是 v，動能便是(1/2)mv2。
等速又好，加速又好，減速都好結果都是相同。