Problem with cats surviving high falls

[russ_watters]

Yeah, I agree. You explained it much better than Russ did (and his mentioning the word "sudden" when talking about the change in acceleration versus the lack of it didn't really help because in fact after the initial point of falling the cat experiences a gradual change in the acceleration/force it feels, till it becomes equal to what the cat feels on the ground, at which point it relaxes).

Yes, sorry, there is a sudden chang at the moment it jumps, then a gradual change as the acceleration decreases near terminal velocity. The whole process only takes a few seconds though.

 

[himanshu2004@]

@Russ
Yes it makes sense, I get it now

 

[harrylin]

I've been reengaging with physics since I'd stopped formally studying it after high school (with the exception of a little bit I we had during studying computer science at university).

So I was revising my concepts of inertial frames, non intertial frames, "fictitious" forces experienced during acceleration etc, and I remembered reading in a book by Hallilday and Resnick that cats survive falls from higher places better than from lower ones (with 4-5 floors being the most dangerous height). I couldn't exactly reason out why it would happen and so I searched it on the web again and this seems to be the reason:

Cats relax after reaching terminal velocity. This causes them to spread out their posture, which reduces their terminal velocity. The further explanation given is that we can feel acceleration (and not uniform velocity).

I have a problem with this explanation, because we should only be able to "feel" acceleration when we are being accelerated by the push or pull, so to speak, of the accelerating frame. This should not happen in when accelerating due to the effect of gravity, because all of our atoms would be experiencing the same acceleration.

So, is there a problem with the explanation, and if so, what is the right explanation?

Thanks!

I've seen another explanation which may be "better"; and I sure think that it's for mechanics even more fascinating.

In a nutshell, when a cat falls it does an amazing rotation act in mid air to orient its feet down - and that takes a little time. It's probably of primary importance to land on its feet.

Just Google for "falling cat" (what did you search for?) Here a few results:

http://en.wikipedia.org/wiki/Cat_righting_reflex

http://www.ocf.berkeley.edu/~barneye/kitty.html

Note: From the images it looks to me that typically cats don't spread their legs out.

Cheers,
Harald

 

[himanshu2004@]

@harrylin
Sorry I should have clarified that the "righting reflex" bit I did understand. However, the physics of falling cats involves both the righting reflex and them relaxing and spreading out more on reaching terminal velocity, which further reduces terminal velocity as result of increased air drag.
My doubts, which have now been resolved, were regarding how they feel they have stopped accelerating upon reaching terminal velocity, because one doesn't "feel" acceleration due to free fall in gravity the same way one "feels" acceleration inside say an accelerating car (which is a non-inertial frame, unlike free fall). More specifically, even though I understood that free fall would feel different from the familiar feeling of being on the ground, it took me a little time to realize that that gradually by the time of reaching terminal velocity the cat would almost "feel" similar again to what it is used to feeling on the Earth (in terms of it feeling an upward trust from below, which is absent in ideal free fall)

 

[rcgldr]

Once you reach terminal velocity you no longer feel like your falling because the resistance of the air is in equilibrium with gravity.

This is the key factor. Once at terminal velocity, the cat is probably not relaxed, but instead trying to grab the air it feels in it's paws, similar to a skydiver using arms and legs to control attitude while at terminal velocity.

Once at terminal velocity, you don't get the sensation of free fall acceleration in your internal organs, because you're no longer accelerating, and the full force of gravity is once again pulling all your internal organs downwards since you're at constant velocity.

Other factors why some cats survive large falls (I don't know how rare survival is), is their terminal velocity is much lower than a humans, and their bone structure may be more flexible while their smaller internal organs may be more resistant to fluid shock.

Rather than dropping a cat, one of those skydiving like chambers that generate a high speed upwards flow of air inside a padded cylindrical tower could be used to see the cat's reaction.

 

[himanshu2004@]

Once at terminal velocity, you don't get the sensation of free fall acceleration in your internal organs, because you're no longer accelerating, and the full force of gravity is once again pulling all your internal organs downwards since you're at constant velocity.

I agree with most of your post, but just a minor correction - the downward push felt has nothing to do with internal organs versus non-internal organs. When on the earth, the downward (or upward depending on how you look at it) force is felt by the all the organs - the body feels this downward tendency to push into the feet, the abdomen into the hips and pelvis, and the head into the neck, etc - ultimately the entire musculo-skeletal structure feels it depending on how were are resting our weight. And hence in free fall, where this effect is absent, the difference would be felt pretty much all through the body.

You probably realize this anyway, but this notion of "internal" organs creates a confusion, as was apparent from some of the posts, where one compares the body's exterior with a non-intertial frame like an accelerating car, and hence assumes that the "internal" things feel inertial forces (relative to the non-inertial frame).

 

[harrylin]

@harrylin
Sorry I should have clarified that the "righting reflex" bit I did understand. However, the physics of falling cats involves both the righting reflex and them relaxing and spreading out more on reaching terminal velocity, which further reduces terminal velocity as result of increased air drag.

OK - do you have evidence of that?

My doubts, which have now been resolved, were regarding how they feel they have stopped accelerating upon reaching terminal velocity, because one doesn't "feel" acceleration due to free fall in gravity the same way one "feels" acceleration inside say an accelerating car (which is a non-inertial frame, unlike free fall). More specifically, even though I understood that free fall would feel different from the familiar feeling of being on the ground, it took me a little time to realize that that gradually by the time of reaching terminal velocity the cat would almost "feel" similar again to what it is used to feeling on the Earth (in terms of it feeling an upward trust from below, which is absent in ideal free fall)

That also sounds wrong to me: why would a cat wait until it stops accelerating for spreading out its legs? It makes much more sense to me if a cat spreads out its legs when it feels a strong air flow. If I were a cat, that's how I imagine I'd react.

Cheers,
Harald

 

[himanshu2004@]

@Harrylin
I don't have any personally collected evidence, but you could see the link posted earlier in the thread and this phenomenon seems mentioned in several places including texts of some repute.
Regarding what you don't understand, I cannot provide any new explanation other than what has already been provided in this thread (in fact this entire thread has been about this). So maybe go through it again, and see if it makes sense. Just to point out, the cat doesn't spread out it legs like making them horizontal or something, but they become more spread out relative to when the cat is uncomfortable and stiffer. The link I spoke of also a picture of that.

 

[BruceW]

You probably realize this anyway, but this notion of "internal" organs creates a confusion, as was apparent from some of the posts, where one compares the body's exterior with a non-intertial frame like an accelerating car, and hence assumes that the "internal" things feel inertial forces (relative to the non-inertial frame).

This is what happens when the body exterior is subject to some contact force from outside. So the comparison is good, (unless the force is gravity, which acts on everything).

 

[himanshu2004@]

@Bruce
Yes that's exactly what I was trying to clarify that acceleration due to gravity is different from accelerating in non-inertial frames, such as cars that we are used to thinking about whenever we think "acceleration". In fact, being on the Earth (and hence NOT accelerating due the Earth's upward thrust) is like being in a non inertial frame which is accelerating upwards with an acceleration g.

 

[harrylin]

@Harrylin
I don't have any personally collected evidence, but you could see the link posted earlier in the thread and this phenomenon seems mentioned in several places including texts of some repute.
Regarding what you don't understand, I cannot provide any new explanation other than what has already been provided in this thread (in fact this entire thread has been about this). So maybe go through it again, and see if it makes sense. Just to point out, the cat doesn't spread out it legs like making them horizontal or something, but they become more spread out relative to when the cat is uncomfortable and stiffer. The link I spoke of also a picture of that.

Thanks I'm perfectly happy with my explanation of the facts (not the suggestions) that I have seen presented.

 

[mender]

I am pretty sure that if the metal box is air-tight, the balloon would fall exactly as the same rate as the box.

Only if the balloon has exactly neutral buoyancy.

 

[himanshu2004@]

Only if the balloon has exactly neutral buoyancy.

No, whatever equilibrium position the balloon was in before the box is dropped, it would more or less continue to stay there, neutral buoyancy or not. (Of course one cannot really talk of an equilibrium position of the balloon during the box's free fall, because that's like being in zero gravity.)
So if the balloon was filled with a lighter-than-air substance and was resting against the roof of the box, it would continue to stay there during the box's free fall drop. And if the balloon was filled with air, water, stones, etc and was resting against the floor of the box, it would continue to stay there.
Note that any effects of buoyancy wouldn't be applicable during the free fall of the box, because there is no concept of weight in free fall.
(For simplicity, I have ignored disturbances due to redistribution in the air's density from being higher near the bottom of the box initially to changing towards uniformity during free fall)

If there is an error in this analysis, please point it out; my Physics is a little scratchy from many years of staying away, but I intend to fix that by re-engaging.

The original poster's thought experiment, and incorrect conclusion, for context:

A crude analogy, imagine an air filled balloon sitting on the floor of a metal box, attached to the side by a length elastic. You suddenly drop the box. The balloon does not fall at the same rate as the box and is likely to hit the roof of the box. It is "left behind" for a short while. It's the same effect in your body.

Your whole body and everything in it may be subject to the same force from gravity, but it doesn't mean it will fall at the same rate whilst in a medium.

 

[BruceW]

Good explanation. Am I right in thinking that astronauts upon re-entry will initially be weightless, then when the spaceship starts to hit the atmosphere, the astronauts will begin to feel some contact with the floor again, and then when the spaceship reaches terminal velocity, the astronauts will 'feel' just like we do sitting on earth. (apart from the fact that they will get very hot).

 

[mender]

Note that any effects of buoyancy wouldn't be applicable during the free fall of the box, because there is no concept of weight in free fall.
(For simplicity, I have ignored disturbances due to redistribution in the air's density from being higher near the bottom of the box initially to changing towards uniformity during free fall)

I was considering the removal of an acceleration field to have similar but opposite consequences as the application because of that effect. When a vehicle with helium balloons in it accelerates, the balloons reposition themselves in the air in the vehicle, responding to the change in air density gradient. For the analogy, that means that unless the balloons are exactly neutrally buoyant, their acceleration rate will differ from the box but only during the redistribution phase.

So we're talking about the same thing, but I was specifying that effect rather than ignoring it. Other than that, there would be no difference when the acceleration field that is acting on all the objects uniformly is removed.

ETA: And now that I think about it further, it appears that you were accounting for that effect as well when referring to the lack of buoyancy effects when in free fall, in that all balloons become "neutrally buoyant" when in free fall. Well thought out and well done! You are correct on all points!

Thanks for the discussion!

 

[mender]

Good explanation. Am I right in thinking that astronauts upon re-entry will initially be weightless, then when the spaceship starts to hit the atmosphere, the astronauts will begin to feel some contact with the floor again, and then when the spaceship reaches terminal velocity, the astronauts will 'feel' just like we do sitting on earth. (apart from the fact that they will get very hot).

Yes; when they reach a constant rate of descent, they will be feeling one g of force holding up their bodies.

This applies to an aircraft at a constant descent or ascent rate; the amount of lift generated by the wings will be essentially the same as when level, contrary to intuitive impressions.

 

[himanshu2004@]

Good explanation. Am I right in thinking that astronauts upon re-entry will initially be weightless, then when the spaceship starts to hit the atmosphere, the astronauts will begin to feel some contact with the floor again, and then when the spaceship reaches terminal velocity, the astronauts will 'feel' just like we do sitting on earth. (apart from the fact that they will get very hot).

Yes, as mender pointed out they will feel the same as they do on Earth as long as the spaceship is ascending/descending at constant velocity (assuming that g is roughly the same at that height). However, I'd like to add that while in all likelihood they descend at roughly constant velocity for large parts, it is very likely not the same as terminal velocity, which is a concept applicable only to free fall against air resistance... unless of course spacecraft s actually adopt the technique of descending in free fall with their engines turned off (till the they need to finally slow down before landing), instead of descending in a more controlled fashion which is most likely what they do.

ETA: And now that I think about it further, it appears that you were accounting for that effect as well when referring to the lack of buoyancy effects when in free fall, in that all balloons become "neutrally buoyant" when in free fall. Well thought out and well done! You are correct on all points!
Thanks for the discussion!

You could say that all balloons become "neutrally buoyant". But its more accurate to say that the concept of buoyancy makes no sense in zero gravity/free fall because there is no concept of weight there.
Yes, it was a good discussion, thank you!