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I was wondering, is this also true in air? If there is an air-tight vehicle that undergoes massive acceleration will the air be compressed to the back of the vehicle? How much acceleration is needed to compress the air?

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- Thread starter Tylemaker
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I was wondering, is this also true in air? If there is an air-tight vehicle that undergoes massive acceleration will the air be compressed to the back of the vehicle? How much acceleration is needed to compress the air?

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Take for example a cubic box 1 meter per side.

Per wikipedia...

At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of 1.2754 kg/m3.

100 kpa = 10197 kg/m

So, if you wanted to accelerate the box at such a rate that the pressure at the rear of the chamber were 2 atmospheres instead of 1 you would have to accelerate it at (10197/1.2754) g. Doing the arrithmatic that would be about 78,352 m/s

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mfb

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You won't get a perfect vacuum, but you get a good approximation if the scale height is small compared to the length of the car. This would require accelerations of more than 10000 g. Possible in ultracentrifuges, but not with a size of 4m.

@physicswinner: Can you show us how you get that number, and where do you use the Born–Oppenheimer approximation? In addition, how is that related to antimatter?

If you just want to post random buzzwords, this might be the wrong place.

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russ_watters

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I don't think the pressure /density gradient plays a significant role: buoyancy does not require or even take it into account.

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mfb

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russ_watters

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Asked another way: for a solid object, does the buoyancy significantly change as it sinks?

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AlephZero

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I don't think the pressure /density gradient plays a significant role: buoyancy does not require or even take it into account.

Buoyant force just requires a continuous fluid, a volume and a direction and magnitude for gravity.

I agree the density gradient is insignificant, but gravity causes a vertical pressure gradient even in a solid object. You can think about buoyancy either way, and get to the correct equation.

Note there may be other causes of pressure gradients, for example in a rotating system. Think about the well known experiment with objects more and less dense than water in a centrifuge, for example.

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russ_watters

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So it propagates by a cycle of the acceleration and deceleration of the SHM.

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