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# Relativity/motion of a river in relation to a fish

Question
Recently, I was watching a science show on TV in which the narrator said that a new idea in physics states that a fish doesn't so much swim in a river (or water) as the water moves the fish. This strikes me in a profound way, but then we have fish swimming upriver to spawn. This was presented as a part of our understanding of relatively, so I wonder if you have heard of this?

Sandra, it is not a deep comment.  The fish pushes backward on the water with its tail.  Some of the water goes to the back direction, of course. But the water pushes the other way (forward) and moves the fish.

A car moves the same way. Its tires press backward on the pavement.  The pavement pushes back, moving the car forward. (The tires are also pressing down on the earth, but that is beside the point. The earth pushes back up, unless you are riding on water!)

Another easy example is when one is with a friend on ice skates in a pond.  If you push him back, he goes back, but equally, you go forward, by your own push.

It is Newton's Third Law: If A pushes B, B automatically pushes A back.

Ben

Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment This really was much clearer than the original information I had. Thanks!!

Relativity

Volunteer

#### Uncle Ben

##### Expertise

I can answer questions regarding Einstein's Theory of Relativity, particularly in Special Relativity. I will not answer homework questions or mathematical problems that require special symbols.

##### Experience

I have taught physics at the college level, undergraduate and graduate, for many years including Special Relativity. I have taught at Johns Hopkins, Case-Western, and MIT. I have also served as a staff member of the Commission on College Physics, which was supported by the National Science Foundation to recommend improvements in the curriculum of college physics departments in the US. I am also the author of a textbook titled Vector Calculus, which was used at MIT in the teaching of electromagnetic theory and relativity. My research interests were mainly in solid state physics, especially the properties of metals at low temperatures. I am listed in the publication known as American Men of Science.

Publications
I have dozens of papers published in the Physical Review and in the American Journal of Physics.

Education/Credentials
I hold a Ph.D. degree in physics from the Johns Hopkins University.

Past/Present Clients
Johns Hopkins University, Case-Western Reserve University, Massachusetts Institute of Technology, Empire State College, Georgetown University, Commission on College Physics, and UNESCO.