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Silly Question

INRM

Philosopher
I
Joined
Jul 24, 2002
Messages
5,505
How come when a plane goes supersonic, the air inside the cabin doesn't go supersonic with it? I mean, it is going faster than the speed of sound too as it's going as fast as the plane...

-INRM
 
THe air inside the plane is moving quickly but only in an external stationary frame of reference. In its own frame of reference, it's stationary and there's no interface between the air in the 'plane and the air outside. Put a hole in the side of the plane and interesting stuff would start happening.

If you were in an airflow moving at say 200mph relative to the ground then the (ground) speed of sound would be around 500mph heading into the airflow and 900mph with the airflow (depending on air density)
 
INRM: How come when a plane goes supersonic, the air inside the cabin doesn't go supersonic with it?
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It does. You are correct that the air inside the plane is going as fast as the plane. As The Don already observed, everything inside the ariplane is going the same speed as the plane, including the pilot, his notebook, the air in the cockpit, the fuel in the tanks, the air inside the tires, etc.

Maybe I have misunderstood your question?

The Don: If you were in an airflow moving at say 200mph relative to the ground then the (ground) speed of sound would be around 500mph heading into the airflow and 900mph with the airflow (depending on <span style="background-color: #ffffcc">air density</span>)
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You misspelled "air temperature". :)
 
xouper said:
It does. You are correct that the air inside the plane is going as fast as the plane. As The Don already observed, everything inside the ariplane is going the same speed as the plane, including the pilot, his notebook, the air in the cockpit, the fuel in the tanks, the air inside the tires, etc.
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Then how come the air doesn't behave as if it's supersonic...

Think about it... air at supersonic velocities behave different than it does at subsonic velocities.

If I walked through the cabin shockwaves wouldn't form off my face and body would it? :p

-INRM
 
Another thing to consider is that the whole Earth is doing about 65,000 mph on its journey around the Sun. Makes the plane's speed seem a bit puny in comparision.

Then there is the Sun's motion within our galaxy to consider and our galaxy's relative motion to other galaxies....
 
INRM said:


Then how come the air doesn't behave as if it's supersonic...

Think about it... air at supersonic velocities behave different than it does at subsonic velocities.

If I walked through the cabin shockwaves wouldn't form off my face and body would it? :p
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Think of it in terms of two people, one riding on a train and another watching from the side of the tracks. Each of them is tossing a ball into the air and catching it over and over again.

From each person's point of view their own ball is just going up into the air and back down again in a straight line. If they were, however to watch the other person's ball, from their point of view the ball would be travelling up and down along something closer to a wave.
 
INRM: Then how come the air doesn't behave as if it's supersonic...
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Because inside the cabin, the air is not moving supersonic relative to the inside of the cabin.

If I walked through the cabin shockwaves wouldn't form off my face and body would it?
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Nope, since your velocity relative to the air inside the cabin is not supersonic.
 
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