# When a rocket or car is accelerating is the air in it going backwards or not?

I'm not sure but when a car is accelerating is the air in the car pushed backwards or not?

If so why did Einstein say that there is no way of finding out (besides tidal effects) the difference between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g.

But if a rocket is accelerating and the air in it is moving like in the car pushed backwards than a balloon is going also backwards in this rocket.

But on the surface of the earth it is not moving (backwards) because the air and the ballon is under influence of the same gravity.

So in this experiment you can see the difference if you are in a rocket or on the surface of the earth.

Probably I miss something but what?

In this video is shown the ballon in the car: http://www.youtube.com/watch?v=y8mzDvpKzfY&feature=iv&src_vid=NblR01hHK6U&annotation_id=annotation_2968066189

http://en.wikipedia.org/wiki/Equivalence_principle#Einstein.27s_statement_of_the_equality_of_inertial_and_gravitational_mass

• The key point you are missing is that the acceleration is constant. If the rocket would suddenly start accelerating like your car, you'd see a difference, but this hypothetical rocket is always accelerating. Feb 17 '16 at 14:18
• Ok, but does the balloon keep standing a little skew ? Feb 17 '16 at 14:20
• I don't fully understand what you mean by this. In a car, if you keep accelerating at the same rate, the balloon will stay in one position, which is a little skewed, yes. Feb 17 '16 at 14:23
• If the balloon is skewed in the car or rocket and not on the surface on the earth than it is possibel to see where you are in a rocket or on the surface Feb 17 '16 at 14:27
• Ah, this is what you mean. It is only skewed in the car because you have horizontal acceleration (car) + vertical acceleration (gravity). In the rocket, it is not skewed (only vertical acceleration) as on earth. Feb 17 '16 at 14:35

You are absolutely correct in thinking that the air inside an accelerating rocket would be pushed backwards. This pushing backwards of the air would create a pressure difference in the air inside the rocket, Making the air at the bottom more dense as compared to the air at the top, very similar to what we find on the earth. This leads to the development of buoyant forces in the rocket and lead to the following results.

As for the balloon, there may be two cases:

1. When the balloon is denser than air:

When the balloon is denser than the air, it 'sinks' in the air and we see the balloon go in the direction opposite to the acceleration (also similar to what we see on earth when a normal air balloon falls to the ground).

1. When the balloon is less dense as compared to the air.

When the balloon is less dense as compared to the air inside the rocket, it 'rises' in the air due to the buoyant forces acting on it (also similar to what we see on earth when a helium balloon rises in the air!) .

Buoyant force also acts in the first case but it is not able to make the balloon float due to the balloon's density being greater than the air's (read about buoyancy).

So, we can easily see that being in a constantly accelerating rocket and being on earth is indistinguishable as long as the acceleration felt on the rocket is 9.8 m/s^2 (apart from tidal forces as you have mentioned).

To develop some perspective : ( i hope this helps clear your doubts)

• But when the balloon is denser than the air wouldn't it has a skew backwards when it hangs on the ceiling while on earht is hangs straigt? Feb 17 '16 at 15:39
• As i have already mentioned that we would see the balloon going against the direction of the acceleration of the rocket. In the case of the earth it would be towards the surface of the earth which is why the balloon hangs in the first place (it tries to go towards the earth's surface but is prevented by the tension in the string of the balloon.) Feb 17 '16 at 15:42
• So isn't that besides tidal effects a way of distinguishing where you are? Feb 17 '16 at 15:49
• No it is not because if you hang the balloon in the same manner in the rocket as you did on earth, you would see that the balloon hangs from the ceiling of the rocket as well( assuming that the rocket is accelerating in the direction of its ceiling). The balloon would exhibit all the characteristics as it would on earth. Feb 17 '16 at 15:51
• And when it is not accelerating in the direction of its ceiling? Feb 17 '16 at 15:55

The behavior of a helium-filled balloon in an air-filled vehicle under the same constant acceleration is the same under all circumstances: Relative to the vehicle, the balloon will move in the same direction that the vehicle is accelerating in, due to bouyancy.

In a rocket, the balloon will migrate towards the front of the rocket, i.e., the direction that the rocket is accelerating in.

In your house, a helium-fill balloon will move toward the house's roof, which is the direction your house is accelerating in. From Einstein's perspective of gravity, a rock that you release in the house isn't accelerated toward the Earth. Instead the rock travels at a constant velocity, and just appears to be accelerating toward the Earth because your house is actually accelerating upward, because the surface of the earth is accelerating upward.

In the video, the vehicle is experiencing a combination of acceleration upward due to the upward acceleration of the Earth's surface, and acceleration forward due to the vehicle's engine. Using a vector addition of the two accelerations means that the vehicle's total acceleration is upward and toward the front, which is the same direction the balloon moves in as much as it can, given that it's tethered.

Technically, the Einstein perspective is that a rock you release doesn't experience any proper acceleration. The rock appearing to accelerate is due to gravity acting as a fictitious force, due to using an accelerating frame of reference. It's the surface of the Earth that's experiencing an upward proper acceleration.

However, saying "it's all relative" like that turns out not to be precisely correct. It's possible to do an experiment to determine that the Earth's surface is indeed accelerating upward, and not just moving at a constant speed. According to special relativity (which is shown to be correct by lots of different experiments), two clocks that are moving at the same constant speed run at the same rate, as is expected. However, if two clocks a fixed distance apart are accelerating at a constant rate, the clock in back runs slightly slower than the clock in front, by a factor of approximately $$T_d \approx 1 + \frac{ah}{c^{2}} ,$$ where $a$ is the magnitude of the acceleration, $h$ is how much in front of the back clock the front clock is in the direction of acceleration, and $c$ is the speed of light. This effect is called gravitational time dilation, but it can be derived just using special relativity, without needing general relativity. The effect is very small, but it's large enough to be measurable with atomic clocks at two different heights. If you do the experiment, you find that gravitational time dilation shows that the Earth's surface is indeed accelerating upward at a rate $a=g$, where $g$ is the acceleration of gravity at the Earth's surface.