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My whole life, I've heard that large accelerations cause damage to humans (e.g. g-forces in space movies).

However, after reading about general relativity, it seems to me that a strong force which affected a body uniformly (such as a very massive object a good distance away) would cause no stresses whatsoever on the human body due to the lack of tidal forces.

It seems to me that the usual stresses of acceleration (like those experienced in a spacecraft) are caused by the fact that the engines push the ship, which must then push the person, causing the part of the person touching the ship to be accelerated at a different rate than the rest of his body.

Is this true? Or would a uniform acceleration on the body still cause stresses?

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    $\begingroup$ A dangerous idea, Brian: don't act upon it. Large accelerations are dangerous because internal organs are not solidly anchored to our bones, causing them to crash into our bones when a strong acceleration is applied to the body. This is how skiers, motorcycle riders, car drivers, parachuters... die. $\endgroup$ – MariusMatutiae Aug 14 '14 at 12:20
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    $\begingroup$ @MariusMatutiae: But that are external forces which do not affect the body uniformly. Parachuters experience 1G acceleration from gravity in free fall, which is an example of the kind of uniform force that's harmless. $\endgroup$ – MSalters Aug 14 '14 at 12:22
  • $\begingroup$ @MSalters True: internal organs don't move in vacuo, but are subject to friction due to adjacent organs, the presence of liquids, and so on. So the effects of some accelerations are actually made less dramatic by friction. But for large accelerations (think of a skier crashing at $120\; km\, s^{-1}$ into a tree, stopping in less than a second) there is no way that this can help you. $\endgroup$ – MariusMatutiae Aug 14 '14 at 12:26
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    $\begingroup$ You keep missing the strong force which affected a body uniformly. The tree is anything but a uniform force. $\endgroup$ – MSalters Aug 14 '14 at 12:27
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If you experience such a uniform force, e.g. when an astronaut on a space walk near the ISS (just earth's gravity), you don't experience any forces at all. That's freefall. Even with 10G, you'd experience a rapid freefall, but that is still harmless.

It's the hitting the ground which kills you - that's not a uniform force.

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    $\begingroup$ It is not possible to idealize a uniform force acting over a body, because the body is not a rigid body (forgive the involuntary pun). Some parts are hanging, suspended from other soft parts: stomach and intestine, liver and gall bladder, the heart. Other parts are delicately resting on a pedestal (the brain) and violent shaking will cause them to crash into our crania. A brisk acceleration of the rigid part (the skeleton) will leave these parts behind, and they will crash into the front or back of our skeleton. Otherwise, why should people feel seasick on airplanes, boats, tilting trains? $\endgroup$ – MariusMatutiae Aug 14 '14 at 18:01
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    $\begingroup$ Same thing as hitting the tree: an external force, in particular electro-magnetic (atoms in your chair repulsing atoms in your body). For the human body, the only feasible uniform force is gravity, and as noted before in freefall your organs don't get squished. $\endgroup$ – MSalters Aug 14 '14 at 18:38
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    $\begingroup$ You are not of uniform density. People in these quickly rotating things (austronaut training, fighter jet), can faint due to large G-forced. I think this is quite comparable. $\endgroup$ – Bernhard Aug 14 '14 at 20:25
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    $\begingroup$ @Bernhard: That's because there are mechanical (external) forces acting on them. There's pressure on your skin keeping your feet up, and by extension your bones up, but not your blood. Gravity on the other hand attracts the blood just as much as any other part of your body. $\endgroup$ – MSalters Aug 14 '14 at 21:24
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    $\begingroup$ @Bernhard In a centrifuge the force is the normal from the seat/platform and it is applied at one boundary of the body, not uniformly throughout the body. MSalters is being very careful about what he is assuming here---and he's right. Most examples that will come to your mind aren't in the category his answer talks about. $\endgroup$ – dmckee Aug 14 '14 at 21:26
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I have a feeling, that you may be asking whether gravity is a force or an acceleration? In the confines of Newtonian mechanics, it's much better to talk about gravity in terms of acceleration, because point-like, free falling test masses responding to a large, gravitating body do not experience any actual forces acting on them. It's only a slight of hand, where we transform F=m*(a + a_gravity) into F=m*a + m*a_gravity = m*a + F_gravity, that gravity starts looking like a force. That's a useful trick for engineering applications, but it misses the actual physics. This, of course, is mended beautifully in general relativity.

As for the question of how to accelerate a human body strongly in a rocket or in an electromagnetic mass driver, (where we do have a real accelerating force)... one can theoretically submerge the body in a liquid of equal average density and fill the lungs with an oxygenated liquid. I do not have any actual research on the limits of this, but I would guesstimate that one can probably survive sustained acceleration beyond 50g with such an approach without undue injury, if necessary. Why one would want to torture oneself in such a way is, of course, another very sensible question. I don't see any practical applications worth pursuing, maybe with exception of "landing" a one way mission on Jupiter (i.e. a pressure vessel that is tethered to a balloon and that could float for months or years in the atmosphere of the planet). Jupiter has a surface gravity of about 2.5g and an astronaut submerged in a tank could certainly survive more comfortably (and maybe longer), even with dry lungs.

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    $\begingroup$ Yes, this comes up as a valid space exploration topic. The solution you offer does increase the maximum sustained acceleration that a person can tolerate. But humans have this tricky need to breathe air, and the lungs will not have equal density. This becomes the limit. To push things further, you'll have to submerge the lungs and carefully evacuate pockets of air in a tremendously uncomfortable process that leaves a human surviving on life support, but might be able to live through much larger accelerations. Short of a nuclear space cannon (I'm serious btw), this doesn't look practical. $\endgroup$ – Alan Rominger Aug 14 '14 at 20:16
  • $\begingroup$ @AlanSE: It's the complications that you mentioned which make this approach of rapid acceleration in spaceflight so questionable. There is no net gain to be had from increasing the acceleration of a rocket (delta v is given by mass ratio and exhaust velocity and is independent of acceleration) and in case of mass drivers, what you win in length by increasing the acceleration, you lose in the mechanical forces that the structure has to withstand, so a shorter, more rapid mass driver doesn't cost less, it may even cost more to build. $\endgroup$ – CuriousOne Aug 14 '14 at 20:19
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In short, you're correct: it's not the fall (uniform acceleration) that kills you, but the sudden stop at the bottom (large contact acceleration). But just for fun I'll point out that it's not clear what "uniform acceleration" even means.

To operationally define (i.e., measure) acceleration you need an accelerometer. An example is a rigid sphere with a smaller rigid ball suspended in the center of it with springs. When the sphere accelerates, the springs compress and expand, and you can read off the magnitude and direction of the acceleration from that. But this only works if the acceleration is via a contact force. If the outer sphere and inner ball are accelerated together by the external force, so that the ball remains at the center, then this device will register no acceleration. The same argument applies to any mechanical accelerometer.

In Newtonian physics that's the end of the story, but in special relativity you can build a better accelerometer using light. Suspend the ball with rigid rods instead of springs, put a mirror finish on the inside of the sphere, emit an isotropic pulse of light from the ball, and time its return. If you are moving inertially it will all arrive back at the same time, but if you are accelerating it will not. This works as long as the acceleration is nongravitational, but fails for gravitational acceleration in general relativity, since that bends light too. [It can detect gravitational tidal forces, but not vector acceleration.]

If you take a hardline view of what "uniform" means, you're forced into the conclusion that uniform acceleration not only doesn't damage you, it doesn't do anything, and ought to be treated as physically equivalent to no acceleration at all. This is one way of understanding the equivalence principle. In GR (unlike Newtonian physics) only gravitation can accelerate "uniformly" in this sense; anything else can be detected by the light-based accelerometer.

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You are correct. The concern over accelerations is with respect to a force applied on the surface of your body. Even with something like a uniform fluid to apply nearly even pressure across the body, your interior will always have density differences. Any density differences will create internal forces when the outside of the body is given a net force.

A uniform gravitational field or any other magical method of creating a force on every part of the body in such a way as to generate uniform acceleration would not be dangerous. It is the fact that we have no way to control such fields that it's not mentioned.

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Basically, it appears that, according to the opinions expressed in the answers in this page, there is a difference between acceleration by a distant gravity field, and acceleration, say, against a wall as we crash with our car.

Let me restate the opinions expressed here in this form: we lock a person inside a windowless spaceship, then subject the spaceship to acceleration by rockets, or free fall into a gravity field. Then according to the opinions here, it is possible to distinguish between the two circumstances by means of experiments carried out purely inside the spaceship.

The statement above contradicts the principle of General Relativity, according to which (in layman's terms) it is impossible to distinguish between these two situations.

Your conclusion is revealed as even more strident by the fact that the OP explicitly mentions a uniform field, so that we are deprived of the only phenomenon (the deviation of geodesics) which can truly distinguish between the two different circumstances.

It also contradicts common experience: astronauts undergo a period of training in a centrifuge, where they are subject to collapses, loss of conscience, nausea, vomit. They are also trained in airplanes in free fall, which are better known by their nickname, the house of vomit. It is revealing, at least to me, that the effects of free-fall or of a centrifuge are indeed the same, except perhaps for the severity of the symptoms; but there is little to be surprised there, given that the centrifuge is not limited to the modest acceleration, $1\; g$, which can be achieved during free fall in the Earth's gravitational field.

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  • $\begingroup$ There is a difference between the way gravity works and the way contact forces works. It really is that simple. Gravity (when uniform) works by accelerating every bit of mass at the same value of acceleration. This is different from a contact force which must be transmitted from the point where applied though the body by other contact forces. That is where the differences in density and elastic moduli come into play. $\endgroup$ – dmckee Aug 15 '14 at 19:28
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You will answer your question if you understand inertia: mass tend to oppose resistance to movement.

Example: In a lift going upwards e.g., your feet are lifted while your head "wants" to remain at the same place (in a galilean reference frame). In the frame of the lift, everything happens as if a force ($m_\text{head}\ddot x$, roughly) was exerted on your head, downwards. The heavier is your head, the greater is this force, which can, of course, damage your poor little body.

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  • $\begingroup$ A lift moving upwards does not exert a force uniformly on your body. In fact, the force will act exclusively on your shoes/feet. $\endgroup$ – MSalters Aug 14 '14 at 23:17
  • $\begingroup$ @MSalters Did I say that it did? $\endgroup$ – anderstood Aug 15 '14 at 0:39
  • $\begingroup$ @MSalters After re-reading the question I understand your point. My answer does not answer the askers question, who asks about a force acting uniformly on the body. Yet, I believe its a educational answer, and that somebody who understood it can answer the original question :) $\endgroup$ – anderstood Aug 15 '14 at 0:43

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