So downstream speed = still water speed+water flow speed.
Velocity = still water velocity-water velocity.
Supplementary explanation:
1, the physical velocity is a relative quantity, that is, the displacement change of one object relative to another object (reference object) in unit time.
2. There is also an average speed in physics: the ratio of the time an object passes through a displacement to the time it takes is the average speed of the object at that displacement, and what we usually say is the instantaneous speed.
3. Usually, when we describe the speed or quantity of actions completed in unit time, we also use speed. Such as typing speed and translation speed.
Extended data:
Average speed:
The ratio of the displacement of an object to the time spent is called the average speed (regardless of any form of motion). It is the ratio of the displacement of an object to the time interval used for the displacement. The velocity formula V = S/Δ t can only roughly reflect the velocity of a moving object with variable speed, and it is a rough description of the movement of the object.
Or the speed at a certain moment. It can also be said that it refers to the speed at which a moving object passes through a certain point or a certain moment. The velocity formula v=ds/dt is a detailed description of the motion of an object.
Baidu encyclopedia-speed