# Relative velocity of river swimmer

1. Nov 21, 2014

### gracy

in swimmer-river physics problems ,velocity of swimmer in still water=velocity of swimmer with respect to ground OR velocity of swimmer in still water=velocity of swimmer with respect to river
which of these is correct .I am really confused.

Last edited: Nov 21, 2014
2. Nov 21, 2014

### Danger

I don't know anything about swimming. In flying, a pilot uses "ground speed" for navigation and "airspeed" for performance characteristics. They're related and both are necessary. I would expect swimming to be similar, since fluid dynamics rules both situations.

3. Nov 21, 2014

### gracy

i am talking in context of swimmer-river physics numericals.

4. Nov 21, 2014

### Danger

Sorry... I just re-read your first post more carefully. In still water, there is no difference between the river and the ground as reference points, so both of your choices are correct.

5. Nov 21, 2014

### Staff: Mentor

In still water, they're the same so are both equally correct. If there's a current, then the problem is incompletely specified if it doesn't say which one is intended.

In some problems, you will be expected to figure out which one is intended from the context.

6. Nov 21, 2014

### sophiecentaur

There is no problem if you use the velocity through the water vector and the velocity of the water vector. The velocity over the ground is the vector sum. How is that confusing? Vectors work that way and sticking to the rules will give the right answer. (As long as we avoid relativistic situations or different geometries)

7. Nov 24, 2014

I have done so many problems like these (river-boat). I have done those from I E IRODOV and S S KROTOV. There I took velocity in still water as velocity wrt river.
Reason : if I am the river, to me I am at rest. To earth, I am flowing. Hence to me the boat moves with the velocity the sailor thinks he is moving....
I have got all answers using this. I was confused initially.IF U HAVE A SPECIFIC QUESTION, I CAN SHOW YOU HOW TO USE RELATIVE VELOCITY CONCEPT.

8. Nov 24, 2014

### gracy

you are right in my textbook also velocity in still water is taken as velocity wrt river.But i am still not getting it' reason.

9. Nov 24, 2014

### A.T.

I guess "velocity In still water" is short for "velocity in the reference frame where the water is at rest", or for "velocity in the ground frame if the water wouldn't move relative to the ground".

10. Nov 24, 2014

### NTW

Always remember the simple rule that absolute motion is relative motion plus dragging motion, where:

Absolute motion => that of the swimmer with respect to the river's bank
Relative motion => that of the swimmer with respect to the water
Dragging motion => that of the water with respect to the river's bank

With that in mind, solving that type of problems becomes very easy.

11. Nov 24, 2014