# Shock Wave relativity

Gold Member
Ok, imagine this experiment:

I am in Mars, inside of a supersonic storm. There is a small sphere standing inside the incident flow, and I am in a laboratory viewing the shock wave caused by the sphere. I have some pressure measurements of the flow, (i.e around of the sphere surfice), so I can demostrate the shock wave presence.

At this point, I have the idea of choosing a different reference frame for extracting measurements. I choose just a reference frame where I can see the flow being completely subsonic (i.e. the laboratory translates at some speed so that viewing is possible, by means of a galilean velocity composition). If the flow is subsonic, no shock wave will be seen at all. I suppose (certainly?) that static pressure and entropy measurements are not a function of the reference frame chosen, so I will have the same experimental figures of the first case, where the flow was supersonic.

I mean, in some reference frame I will see a shock wave, and in another I will not see one. But we know that a fluid throug shock waves have an increasing in its entropy and a strong pressure loss. This two properties are not a function of a reference frame, because are local thermodynamic properties of the flow.

What happens? Is there a shock wave? How could I demostrate that in these two laboratories?

drag
I believe you got yourself a bit confused by this one,
happens to me too.

In a "different" reference frame the relative speed between
your sphere and the gas will still be supersonic and that's
ALL you need to look at. As for the movemnt of the
gas itself - it doesn't matter weather an object moves
through the gas or the gas past the object - on a
windy day you can see that a bird can stay in one
spot or a model airplane can fly very slowly upwind and
very fast downwind - this is because the amount of
lift is constant, and lift depends on RELATIVE motion between
the geometry creating lift - object, and the gas(/liquid).

Live long and prosper.