# Effect of very high accelerations on objects

1. Jul 5, 2013

### kaikalii

What happens to objects (of any material, density, mass, etc) at both the macroscopic and atomic scales when under the effects of very high accelerations, say, 100 g's? 1000 g's? 1000000?

2. Jul 5, 2013

### Simon Bridge

Depends on the circumstances ... high accelerations generally put a lot of stress on macroscopic objects.
Think in terms of the effects of a very large gravitational field.

3. Jul 5, 2013

### A.T.

A large gravitational field is actually a good example why coordinate acceleration doesn't necessarily put a lot of stress on objects. A large gravitational field is locally uniform, so it accelerates objects without causing internal stresses. It's not the amount of net acceleration, but how non-uniform the applied forces are that determines how much stresses there will be.

4. Jul 5, 2013

### kaikalii

But wouldn't large amounts of non-uniform acceleration still have certain effects? I'm pretty sure a human would not be doing too well in a rocket accelerating at 980 m/s^2. But are you saying that the rocket itself would be fine structurally, as it's acceleration is uniform?

5. Jul 5, 2013

*uniform

6. Jul 5, 2013

### Simon Bridge

I was going to say ... :)

@A.T.: Put a beam between two supports... what is the effect of the gravitational field strength on how much the beam bends?

7. Jul 5, 2013

### magaszag

Why use localised gravitation fields of large energy patterns?
Seems to me that is this force weakest of all to keep structural integrity...

8. Jul 5, 2013

### A.T.

No. I was talking about the effects of an uniform gravitational field (constant over space). You are confusing it with uniform acceleration (constant over time). A uniform gravitational field doesn't cause internal stresses, regardless how much uniform acceleration it causes.

9. Jul 5, 2013

### A.T.

The beam bends because the support forces are non-uniform.

10. Jul 5, 2013

### Simon Bridge

But the gravitational field is constant! So you are saying that when the support forces are not uniform, you can[/i get stresses in a uniform gravitational field? i.e. the amount of stress depends, in combination, upon the geometry of the object(s)/system and the strength of gravity?

All other things being equal, the beam experiences more stress in a stronger gravitational field?

11. Jul 5, 2013

### Simon Bridge

Not "localized" : "local".
Gravity is locally constant because you can always find a small volume where F=mg works everywhere in that volume (differing values of g of course). I compare an acceleration to gravity because, locally, you cannot tell them apart. It's a central principle in General Relativity[1]. I also did this because I thought it would help you think about the consequences without me having to make a big list of things that get affected.

Under a spacially constant field, a classical rigid body evenly supported will experience no stress or deformation - in IRL bodies are not rigid and are not strictly evenly supported. We could model a solid as a lot of small masses connected by stiff springs in a lattice. Imagine all the base masses are evenly supported - say it's sitting on the "floor" - the entire structure would still compress under acceleration right?

You will have heard of the possibility of people turned to jelly by very high accelerations?

Can a force have "structure" to keep the integrity of?

The technical way to compare the strength of forces is via "coupling constants"...
http://hyperphysics.phy-astr.gsu.edu/hbase/forces/couple.html
... in that context, you are right: gravity is the weakest. I was interested in how you were thinking about "strength" though, so I could better answer your question.

Another thing that may help is if you told me which particular properties you are interested in - you asked a pretty broad question there, covering thousands of possible effects.

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[1] You don't have to use GR to treat acceleration - though for extreme accelerations that may be more useful: see -
http://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html

12. Jul 6, 2013

### A.T.

That's is self contradictory. In a stronger gravitational field the non-uniform support forces on a static beam must be stronger too.

13. Jul 6, 2013

### Simon Bridge

So it does not bend more in a stronger field?
Is that what you are saying?

14. Jul 6, 2013

### A.T.

It bends more if the non-uniform forces are greater. That's what I'm saying.

15. Jul 6, 2013

### Simon Bridge

The non-uniform forces, in this case, are the reaction forces at the supports.
They must be greater in order for the beam to be stationary in the accelerating frame.
i.e. the reaction forces are proportional to the weight which is proportional to the strength of gravity.

Presumably I can also make the stresses change by changing the position of the supports.
However, we usually try to change only one thing at a time in experiments.

So - I have shown that objects can experience stress under locally uniform gravitational fields, and that the amount of stress depends (amongst other things) on the local strength of gravity. Generally, objects are not uniformly supported throughout their structure - so

So I have successfully shown you what I meant when I said

You appear to agree, so I don't understand your objection.
Is it merely that you wanted to point out that there are other factors in play besides the acceleration?

(Note: the "all other things" that remain the same, which you found "self-contradictory", referred to the description of the thought-experiment setup - keep that the same, change the gravity, then the reaction forces are different, the beam bending is different, there will also be some compression in the supports etc.)

16. Jul 7, 2013

### A.T.

Maybe I misunderstood what acceleration you mean in regards to the large gravitational field. I was explicitly talking about the coordinate acceleration of a free falling object. This acceleration doesn't cause any stresses.

17. Jul 7, 2013

### Simon Bridge

Ahh there you go, all cleared up :) Free fall is a special case of acceleration under gravity.
I was citing the equivalence principle.
http://www.einstein-online.info/spotlights/equivalence_principle

Last edited: Jul 7, 2013
18. Jul 7, 2013

### BruceW

As A.T. has been getting at, if the entire rocket (and the person inside) are all undergoing a huge uniform acceleration in space, then the person would not feel anything, and the rocket would be structurally fine.

However, if the acceleration is being caused by the exhaust of the rocket, then the person would be pushed into his seat, and the rocket might break if it drove its 'engines' too hard.

So it is important to state what is causing the acceleration. What the person feels will depend on what is causing the acceleration.

19. Jul 7, 2013

### skeptic2

It's easy to get an idea of what 100s or 1000s of g's will do to an object simply by dropping it on a very hard surface. If you can measure or calculate how much both the object and the surface it was dropped onto were deformed, you should be able to calculate the acceleration.

For instance suppose you dropped a bar of soap in the shower from a height of 1 meter resulting in a deformity (dent) of 3mm. You could calculate the acceleration the soap bar experienced was approximately equal to 333 g's.

Last edited: Jul 7, 2013