1

New Theories / Re: Was the light speed problem really solved by Einstein in 1905 ?

« on: Yesterday at 12:45:43 »

The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.

https://en.wikipedia.org/wiki/Doppler_effect
From the definition above, Doppler effect can happen if wavelength changes even if the frequency stays the same, which means that the propagation speed also changes accordingly.

I agree that the definition above says that, but it is wikipedia, and I think they mean frequency change and associated wavelength change and not the case of wavelength change associated only with refraction and not frequency change.  I'm saying wiki is wrong here.

From oxford dictionary (top of list if you google "what is doppler effect"):
"[physics:] an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other. The effect causes the sudden change in pitch noticeable in a passing siren, as well as the redshift seen by astronomers."

Britanica:  "Doppler effect, the apparent difference between the frequency at which sound or light waves leave a source and that at which they reach an observer, caused by relative motion of the observer and the wave source."

webster:
"a change in the frequency with which waves (as of sound or light) from a given source reach an observer when the source and the observer are in motion with respect to each other so that the frequency increases or decreases according to the speed at which the distance is decreasing or increasing"

http://physics.bu.edu/~duffy/py105/Doppler.html
"The Doppler effect describes the shift in the frequency of a wave sound when the wave source and/or the receiver is moving."

Pretty much every place except wiki says it's a frequency shift and does not consider a wavelength change without frequency change to be an example of Doppler effect.

2

Physics, Astronomy & Cosmology / Re: What is the cosmic microwave background?

« on: 12/10/2019 01:45:24 »

The conversation seems to imply the CMB generation was quite a short phase (when the universe was a 3000k plasma). If its effects are still saturating the universe after 13.7bn years (due to its photons filling cubic meters of space as fast as they leave it), then the volume of this plasma must have been.... 'large'. But, why dont we see other radiation that must have been emitted before and after this 3000k period?

There was light before then, but the universe was opaque, so the light never got far before hitting something.
There is light since then.  That's all the stars and whatnot, most of them brighter than the CMB.

Why isn't this still filling up cubic meters of space as fast as they leave it like the cmb? Or is this the dark matter that ppl are still looking for?

It does fill space as fast as it leaves it, else the stars would not shine continuously.  There's nowhere you can be (except in places like clouds that obstruct light) where galaxies cannot be seen, so their light fills all space, same as the CMB.

3

Physics, Astronomy & Cosmology / Re: Do half lives of radioactive materials affected by gravity potential?

« on: 10/10/2019 13:58:46 »

How do we reconcile these two different results?

What two results?  The H-K thing wasn't measuring radioactive decay.  It used atomic clocks.  It also did not involve anything in orbit.
The orbital test just involves speeds so low that using a radioactive sample as a clock lacks the precision needed to distinguish the moving sample from the base rate.

The experiment has already been done with the muons, which have a half-life of about 1.56 usec, enough time for light to travel about half a km.  Yet due to time dilation, most of them reach Earth's surface about 15 km away.

How to make sure that the muon detected at sea level is indeed the one formed at 15 km altitude, not above or below it?

The're not all formed exactly at that altitude. Some are higher, some lower, but they don't form at any significant rate at lower altitudes, say 5km.  So they compare the flux on say a 2 km mountain and that at sea level.  Without dilation, the sea level flux should be about 5% of that measured on the mountain.  But the rate measured at near sea level was 73% of that of the higher altitude rate, yielding a relativistic dilation factor of around 8.8

The figures above were taken from the Frisch–Smith experiment, 1963, performed on Mt Washington and Cambridge, MA.

4

Physics, Astronomy & Cosmology / Re: Are gravitational waves faster than light?

« on: 24/09/2019 18:19:46 »

I know not of the podcast in question, but an event like a supernova has the light being delayed by the shock wave needing to travel to the surface of something like a star going supernova.  The light is also not subsequently traveling in a vacuum.  Gravity waves on the other hand are unimpeded by any of this and get here first.  So do neutrinos, which don't even travel at light speed, but fast enough that light (given the initial delay) never manages to catch up.

5

Physics, Astronomy & Cosmology / Re: Why are my atoms not flying apart? Please Hurry - May Have Limited Time Left

« on: 21/09/2019 12:55:04 »

So (I'm sure you see where this is going), if the forces have remained constant, and the distances, however infinitesimally, are increasing, then at some point in the expansion of the universe, won't atoms cease to exist?

The small distances are not increasing.  A water molecule has the same distance between the various atoms as it did billions of years ago since that's the distance where all the forces are in balance.

Likewise, if Earth is pulled a millimeter away from the sun due to expansion, it immediately falls back down to its old orbit since it does not have the energy (kinetic + gravitational potential) to orbit at the larger radius.

6

Physics, Astronomy & Cosmology / Re: The Moon from Earth 4 billion years ago?

« on: 15/09/2019 19:26:01 »

What would moon have looked like from Earth on a clear evening, 4 billion years ago? Was that not shortly before life formed on Earth?

Exact dates for both are not known, but it seems if life was barely there before the Theia event, that event would not have been kind to it.  Maybe..  Life in general is impossibly resilient, and if such an impact should happen today, life would most likely survive it.  Maybe life came from Theia itself.  Think about it.

It would have been extremely close! Immediately after it's formation from the Thea collision it must have resolved to a closest orbit, and moved away over time due to tides over millions of years?

The third of Kryptid's links gives distances, but oddly enough, not length of day or month.  I've heard the day would have been around 10 hours (and will be again if the sun can be prevented from swallowing us before then).  The calculations can be done with conservation of angular momentum laws.

At 4 billion years ago say 200 million years into it's life, the moon also had an atmosphere, coming from it's rocks and craters and internal sourced gases. What were those gases and what colours would they have been? And they would have been blowing in the solar wind, right? Under a dimmer colder sun?

Solar wind might have blown that atmosphere away almost as fast as it was emitted from the rocks.  Unclear if any appreciable pressure ever built up, but your subsequent link says yes, perhaps around 1% Earth's current pressure.

The Earth at this time along the equator, would not have had any land not swept over by the tide during the week?

Oceans took a long time to form after the Theia event.  Don't think tides ever swept clean over the continents.
I've heard of weather forecasts for back then: Hurricanes with mercury pressure down to about 15 cm, strong enough to polish the landscape down to smooth rock.

How many hours would a lunar orbit have taken back then?

Right after the moon formed, less than a day.  Several hundred million years later, maybe 4 days.

The point of centre of the two objects rotating would have been inside the Earth's diameter?

It still is, about 75% of the way to the surface.  Yes, it would have been deeper in back then.

Centrifugal forces would not have balanced out like with the Sun's gravity and the Earth's orbit?  Or is there a solar tide?

Don't know what you mean by centrifugal forces balancing out.  I think the moon's acceleration on Earth might have been as strong that that of the sun at some point.  It isn't close anymore.  The Earth always accelerates towards the sun even when the moon is full, pulling hardest in the opposite direction.  Yes, there have always been solar tides.  You're very aware of them if you live at sea level.

7

Just Chat! / Re: Is there a universal moral standard?

« on: 11/09/2019 04:15:17 »

Yes it is about universal morals. And yes, the situation was designed to show that moral judgement is closely related to knowledge and uncertainty.
Unfortunately, cuteness is not a universal value. Something cute to someone might be not cute for someone else.

Agree with all of this.  Suppose we have full knowledge of the situation.  We have the uncertainty if you want it, like an even chance that cutting a wire will halt or blow the bomb.
What we don't have is the worth of what we're saving. The universe places no worth on anything.  Maybe the building is full of 50 people that would die, or maybe 50 spiders. Are humans worth more than spiders? To humans, sure, but to the universe?

To determine what's the universally most moral action in a particular situation, we need first to determine what's the universal goal we want to achieve, and then calculate and compare the expected results we would get by taking available actions. We should take actions expected to get us closest to the universal goal.

Agree with this if a universal goal can be found, but I don't think there are objective goals. I absolutely agree that the goals should be considered first. What's good for one goal is not so good for others. The Catholic church's stance on birth control for example seems designed to bring about the demise of humanity in the shortest possible time. They don't seem to consider long term goals at all, or are counting on forcing God's hand, like that's ever worked.

Someone might have good intention when making a moral decision, but their decision may produce undesired result if it's based on false information, such as swapped wire of the time bomb.

That part seems irrelevant since it cannot be helped. A person cannot be faulted for having good intentions and attempting what seemed best. It seems irrelevant twice because if he chooses to cut no wire, everybody in the building still dies, so the wrong choice just takes out our hero, but nobody else that wasn't already doomed. I think he'd not forgive himself if he didn't try, but only if attempting the disarming was the right thing to do in the first place, and we haven't determined that.

Here is some possible scenarios which can bring you to above situation.
- You are hired by a building contractor to destroy an old building so they can build a new one. You just get the date/month wrong, perhaps you and your client used the different format.
- You are a national secret service agent ordered to destroy their enemy's headquarter. You are discovered by enemy guard when you tried to sneak out.
- You are a mercenary hired by a terrorist organization to destroy their enemy's economic center. You are waiting to get payment confirmation.
- You are a voluntary member of a terrorist organization to destroy their enemy's economic center. You are willing to die to execute the job.

In all 4 of these cases, you're taking your orders from your employer. You have a goal, and it isn't a universal one. You do your job. If you work for someone you find immoral, then you know you're helping them do immoral acts. Most terrorists/soldiers don't consider their acts immoral.

Alan (post 2) brought up morals coming from your peers, and all the above are examples of that.
I think we need some examples where your chosen peer group has nothing to say, and you're actually faced with wanting to do the right thing and not what some group to which you belong wants you to do.

8

Just Chat! / Re: Is there a universal moral standard?

« on: 09/09/2019 19:05:58 »

Here is some examples to demonstrate that moral judgment is closely related to knowledge and uncertainty.
You are in a tall and large building, and find a massive time bomb which makes it impossible to move before disarming it first. You can see red and blue wires on the detonator, and a counting down clock showing that there is only 2 minutes left before it explodes. You are an expert in explosives, sou you know for certain the following premises:
- if you cut the red wire, the bomb will be disarmed.
- If you cut the blue wire, the bomb will explode immediately, destroying the entire building and killing thousands inside.
- If you do nothing about the bomb, the timer will eventually trigger the bomb.
Which is the most moral decision you can take, which is the least moral, and why?

If this topic is about universal morals, then the bomb question has not nearly enough information. Why mess with a device that has a clear purpose? It has not been stated that there is a goal to preserve the building. Maybe the bomb was put there by a demolition crew who was paid to take it down.
Suppose the building is full of puppies. It is a universal law that it is bad to damage something cute, correct? If so, you've already begged your answer. If not, how am I to know what to do with the bomb even if it has a simple 'off' switch available? The universe seems to provide no input for the situation at hand.

9

Physics, Astronomy & Cosmology / Re: Does the Fermi paradox suggest that superluminal speeds are unachievable?

« on: 15/08/2019 12:03:41 »

While pondering the existence and consequence of faster than light particles, I wondered if tachyons are real and useful then we can travel the galaxy in years rather than in millennia.

How would the reality of tachyons help you (or another civilization) get anywhere fast if you're not made of them?

However, as Enrico Fermi pointed out in his famous lunch time paradox, we have seen zero proof of alien contact. So, either advanced extraterrestrials don't exist or exceeding light speed in space travel is impossible. Any thoughts on this?

You suggest the only solution for the Fermi paradox is the light speed limitation.  Here we are such an advanced civilization and the fastest speed we've been able to achieve is under 0.0001c.  OK, I think we got a ship to go faster than that, but only by dropping off a cliff, not actually having the ship do it under its own power.  The speed of light hasn't exactly been a barrier to us yet.
Contact with aliens doesn't necessarily entail travel.  If they were out there on every 1000th planet, we'd notice.

10

Physiology & Medicine / Re: Why Is It Easier To Complete A Puzzle After A "time-out" ?

« on: 13/08/2019 15:45:34 »

As a sheepy during my morning commute on the train, not only do I listen to the Beach Boys "Sheep Sounds" ewesing my "BLEATS"  but I also do word puzzles !...I have found that when I become stuck on a puzzle and then put it away till the commute home that I can then instantly comBLEAT the once offending puzzle from the morning with ease !

I get the same effect.  I suspect certain assumptions act as mental blocks pulling WOOL over your eyes, and giving it a rest allows these blocks to evaporate.  Looking at the problem anew after a time forces one to reassess what's going on, and often that assessment sees new things that were assumed not there before.  Something like that.

Somewhat related:  I often can't think of a name of a word, person or movie or something, and the harder I try, the more impossible the task of recollection.  Later on, I casually need the same word and it pops effortlessly to mind.  The explanation I give above seems not very relevant to that effect, but both involve setting the problem aside.

Sleeping on it works because your ability to think is severely impaired if you need sleep.  I learned in school not to cram all night, but to get proper sleep before an exam and possibly review the material an hour before the test.

11

Physics, Astronomy & Cosmology / Re: Do we go round in circles?

« on: 10/08/2019 14:16:49 »

I’ve put together a few questions, and made some tentative moves towards possible answers, or where they might be found.  Still a long way to go, but comments would be appreciated.

I put in some comments, but have not read the whole thread.
 

1. If infinity is not a number, how can you subtract anything from it?

It isn't meaningful to subtract a number from a not-a-number.  'Infinity' is more of an adjective, meaning 'without limit'.  Much confusion arises when its syntactic usage as a noun leads one to treat it as a number.

2. If the Universe is infinite, it contains an infinite number of galaxies.  How does one define an infinite number?

Again, it simply means there is no limit to the number of galaxies.

3. If one is subtracted from an infinite number of objects, is the remainder still an infinite number?  If not, what is it?

Per point 1, it is not meaningful to do addition and subtraction with 'without limit'.

4. Would an infinite number of (identical) objects contain all the examples of that object that could exist?

If they're identical, how is it not one object?  This sort of gets into the law of identity.
Perhaps you mean something like 'just because there are infinite points along a line doesn't mean that there are not other points that do not fall on that line.  So there are examples of points not in that set, but I'd not call any of the points 'identical' since they're all at different places on the line.

5 Is “absolute infinity” (sensu, Cantor) amenable to mathematical manipulation?

6.

The Absolute Infinite (symbol: Ω) is an extension of the idea of infinity proposed by mathematician Georg Cantor.
It can be thought as a number which is bigger than any conceivable or inconceivable quantity, either finite or transfinite.

How could this concept be expressed without referring to infinity as “a number”?

Since it isn't a number, not sure what you're asking.  It's expressed as Ω, but that's not your question. One infinity isn't larger than another since they're not numbers.  Perhaps it means 'highest cardinality', but cardinality isn't an expression of the magnitude of something.  Only numbers have magnitude.

I'd not trust wiki on this.  They call it a number, but any number is finite.  I think a proper mathematician would not word the description this way.

An infinite universe is possible only if the mean density of matter in the universe vanishes.

Why?  Simple explanation, please.
...
I'd really appreciate some guidance with #23.  I'm trying to tie up loose ends.

Since nobody replied to this, I'll just say that the mean density of matter in the universe is typically presumed to be the same as it is in the parts we see (cosmological principle).  yor-on's statement would only apply to a model where there is infinite space, but there is finite matter, presumably all clumped nearby.  I know of no such model that is seriously considered.

12

Technology / Re: Can we use physics to detect when people look at security devices?

« on: 10/08/2019 02:09:38 »

Responders are making assumptions that have noting to do with the post.

The responses mostly seem to be on topic, although there is a responder spouting nonsense.

For simplicity I will call a device that uses the double split experiment to detect if someone has percieved the device a "quantum perception switch" or QPS.

What exactly do you suppose this QPS does?  If it watches the door of the room, then it detects the person directly, and the double slit setup seems to not even be required.  So it is watching only the double-slit setup.  Exactly what would distinguish the experiment running with or without an observer?  You can win a Nobel prize if you can answer that.

If I put a QPS attached to the ceiling in the middle of a secure room would it send security a message any time somebody enters the secure room?

Only a Schrodinger's box is secure.  I'm willing to posit one, even though the best one ever built barely managed to contain an object just large enough to see without instruments.

Would a cat activate the QPS?

Would a camera actiate the QPS?

Depends on what it is set up to detect.  You haven't told anyone this.

13

Physics, Astronomy & Cosmology / Re: What is the nature of dark matter around a black hole?

« on: 05/08/2019 12:03:47 »

What is the nature of dark matter around a black hole? Does it fall in? Is there dark matter in black holes? Is it repelled?

If it (a specific bit of it) is aimed quite perfectly, it will fall in.  More likely it picks up speed as it falls and exits the system similar to the way a comet leaves.   Maybe it orbits, or maybe it moves too fast to orbit.
It cannot be slowed or repelled since there is no way in interact with dark matter.  That's what makes it dark.

14

Physics, Astronomy & Cosmology / Re: why are some heavenly bodies tilted on their axis and not others?

« on: 24/07/2019 11:51:59 »

Our moon is tide locked with Earth, and thus has lost most original spin/tilt it might have had.
Looking it up, the moon seems to have an inclination (tilt) of about 1.54° relative to the ecliptic and an inclination of its orbit of about 5.15°.

The inclination is typically fairly random and the odds of it being exactly zero is pretty remote.

15

Physics, Astronomy & Cosmology / Re: The Ultimate Special Relativity Question to Help Amateurs Understand

« on: 19/07/2019 19:58:15 »

You need to change your topic title per the first reply, and also because the OP has a lot of questions, none of which are the 'ultimate' one.

This scenario has been written to assist in the understanding a multitude of aspects of special relatively for the amateurs of us interested in cool physics (including me). I would be grateful for a physicist with a University degree to answer in detail (sorry no half baked musings on this on please).

While my answers are not half baked musings, I doubt there are any actual relevant physicists on forums like this one.  You want an educated answer, fine, but it's not going to come from a physicist.

Scenario:
C         <-- 100 million light years -->   A          <-- 100 million light years -->    B
                                             1 --> (a = 20 m/s/s until 0.99999c)
                                       2 --> (a = 20 m/s/s until 0.5c)
                                  <-- 3 (a = 20 m/s/s until 0.99999c)   
                                  <-- 4 (a = 20 m/s/s until 0.5c)   

Primary Details:
- Spaceships 1 and 2 set out from observer A towards observer B who is 100 million light years away. Each accelerate at 20 m/s/s (i.e. ~2g) until cruising velocity is made. 1 accelerates to 0.99999c while 2 only accelerates to 0.5c
-Similarly spaceships 3 and 4 set out from observer A to C.

It isn't really stated, but I presume from this description that C, A, and B are locations in some reference frame and that the observers so named are stationary at those locations and the distance given (the 100m LY) is measured in that frame.
The fact that you have accelerating ships kind of gets into accelerated reference frames which is covered better under general relativity (GR), but special relativity also covers it.  It just requires more calculus than would something like instant acceleration scenarios.

The pairs of ships accelerate side by side until the slow ones quit and the faster ones continue to accelerate up to the higher speed.

- Each spaceship also decelerates at 20m/s/s as it approaches the target observer such that they stop when they reach the target.

- 1, 2, 3, 4, A, B, C are all fitted with 6 lasers each, directed to each of the different observers and spaceships. The lasers flash once per 100 seconds for 1 second duration. i.e. on the 99th second the lasers turns on for 1 second. The light is green (wavelength of say 532 nm).

Secondary Details for Auxiliary questions :
- Each laser makes 1 GW of effective light emitted during the 1 second bursts.
- Each spaceship weighs 10 Tonnes excluding fuel. Let's assume each ship has some sort of particle accelerate to use as thrust that can accelerate the particles used as thrust to ~0.9c. Also it is powered by let's say some theoretical antimatter/matter source and the whole system can convert all matter into thrust at 99.9% energy efficiency.

If it exhausts the reaction mass at 0.9c, it is 90% efficient by that metric.  It really doesn't make a different if it is antimatter drive or water rocket like the space shuttle.

Please ignore the expansion of the universe.

It wouldn't be SR if we assumed otherwise.

Q1.a) According to spaceship 1, how long did it take to reach B?

~447222 years

b) According to spaceship 1, when does spaceship 2 arrive at B?

When the ships rejoin, the clock on ship A reads about 100446222 years.

c) describe, inducing a rough timeline, what spaceship 1 observes from the laser lights (i.e. the wavelength, how long between flashes and duration of flashes, the energy intensity) , originating from A, B, 2, 3, 4 as it accelerates to 0.99999c, cruises at 0.99999c, and finally approaches B.

The flashes are all the same at first.
Once everything is up to cruising speed, the light from A is dilated by a factor of about 223.6 and a Doppler factor of 100k, so it blinks around 22.36 million times slower.  Light from B is dilated by 223.6 again, but Doppler is in the other direction, so it blinks 447 times faster.  Frequency shifts are also proportional.

To do one more, the fast ship 3 has a gamma of about 1e5 and a Doppler factor of about 1e10 so it blinks about 1e15 slower.

a) According to spaceship 2, how long did it take to reach B?

I get 173 some million years

b) According to spaceship 2, when does 1 arrive at B?

Frame dependent question since spaceship 2 is not present at that event.

describe, including a rough timeline, what spaceship 2 observes from the laser light originating from A, B, 1, 3, 4 as it accelerates to 0.5c, cruises at 0.5c, and finally approaches B.

At cruise, gamma is 1.1547 and Doppler is a factor of two, at least relative to the lettered objects.  It can be worked out from that.
Ship 1 is moving at .99987c so those two ships see blinking slowed from each other by about 6.2 million

]a) According to observer B, when does 1 reach it?
b) According to observer B, when does 2 reach it?

We haven't stated what the clock at B reads at any particular event, so this cannot be known.  If it is synced to A in that original frame, then ship 1 gets there at year 100001000 and ship 2 at 200MYr (plus about 100 days).  This is pretty trivial arithmetic.

c) Describe, including a rough timeline, what observer B observes from the laser light originating from 1, 2, 3, 4.

Same thing that those ships saw when observing B.
As for timeline, B will observe nothing for 100m years since the departure event takes that long to be seen.  So ship 1 appears to be in flight only the last 1000 years, and ship 2 for the 2nd half of the duration.  All 4 ships appear to get up to cruising speed relatively quickly.  20 m/secē will definitely get you there.

a) According to observer A, when does 1 reach B?
b) According to observer A, when does 2 reach B?

Assuming the frame of A and that clocks are synced in that frame, all these events are the same answer as B's answer.

c) Describe, including a rough timeline, what observer A observes from the laser light originating from 1, 2, 3, 4.

Same as those that seen by the ships looking at A.

d) According to observer A, how fast are 1 and 3 moving away from each other?

That's a 3 way relation, not particularly defined.  Ships 1 and 3 are moving at .99999999995c relative to each other period.  It's an objective relation, not a frame dependent one.

b) If each laser had  a narrow enough focus and the observers had a large enough receiver to capture all of the light emitted to them from each spaceship, what would the energy levels (GW) look like from each spaceship.

Work it out as a function of energy conservation.

16

New Theories / Re: The possibility of coming to life again after death

« on: 19/07/2019 15:42:09 »

What I mean life doesn't continue forever,  it is discrete periods of living and dying, each period starts and ends, in a way that I won't feel nor remember its continuity but I still exist and feel life itself forever.
It is just the continuity of enjoying things eternally  without feeling eternity.

You should just adopt a more mainstream philosophy that meets these goals.  Try eternalism, which lets you do just that: continuity of enjoying things eternally, but without all the funny need to posit an implausible immaterial experiencer or the need to posit a cyclic universe that somehow produces the same identical configuration of matter each time.

17

Just Chat! / Re: An eternal quandary

« on: 18/07/2019 21:49:39 »

Normally not much, but being its primary defense, I suppose it would chuck plenty of wood at the bear that's with you.

18

Physics, Astronomy & Cosmology / Re: How can galaxies collide if they're moving away all the time/

« on: 18/07/2019 04:10:56 »

Of course variances can be generated by gravity ie bigger mass affecting smaller mass, but to have galaxys vectors significantly altered would take alot of mass.  Perhaps the blackhole and where it goes to is capable of pivoting galaxies ?

Black holes seem to play no significant role.  The central black hole of our galaxy is only about 3% of the mass of the galaxy.  The (relatively nearby) great attractor is on the order of hundreds of times as massive, plenty to pull us and Andromeda together.  There is another even larger mass (called the Shapley attractor) beyond that.  I think that's it, and beyond that scale, mass distribution becomes more or less homogeneous.

19

Technology / Re: What is the impact force of dropping a 40kg weight from waist height?

« on: 14/07/2019 20:11:33 »

Floor loadings are complicated.
The lab where I work has a reinforced concrete floor and it's rated for something like 3 tonnes per square metre. (If I remember rightly).

But, I am 75 Kg or so, and I don't go through the floor, even though, as I walk, most of my weight is carried on a few square cm of my toes.

They had airplane cabin floors rated (local) for 100 tons per square meter, but these women would go on with perhaps 50kg mass and heels that were one sq cm which works out to half a million pascals, about 2.5 times the rate on the deck.  They'd punch holes in them.  They had to ban the shoes on some airlines.

Say its a strip under my shoes an inch by 4 inches that holds up my weight as I walk.
That's about 1/400 of a square metre, so it should hold 1/400 of 3 tonnes
3000 Kg/ 400 gives 7.5 Kg and I'm about 10 times that.

As you point out below, your 3 tons/meter is more of a larger scale average.  The airplane obviously isn't going anywhere if actually loaded to 1% of that rated average pressure.

Ther are, in effect, two different "maximum" floor loads.
There's one where the floor supports fail, and there's one where the material fails locally.

Indeed, and there can be even more ratings, all at different scales.

20

Technology / Re: What is the impact force of dropping a 40kg weight from waist height?

« on: 14/07/2019 19:49:51 »

Any small attempt to explain my mistakes would be gratefully received.

OK....

So you need to know the stopping distance in order to calculate the stopping distance?!!

The question was about computing force.  Measuring/computing stopping distance is one way to begin to approach this.  Nobody suggested needed to know X in order to calculate X.

The stopping distance is misleading.  The floor is going to distort some distance, and the object hitting it will also distort.  You seem aware of this in your comment below.

The smaller the distance for deceleration, the larger is the force, so if the weight stops in 0.5 millimeters the 1Kg becomes 2,000Kg or 19620 Newtons.

For a point mass decelerating uniformly over half a mm, 19620 Newtons under Earth gravity, yes, but deceleration is not uniform over that distance.  Force will vary, and thus hit a max value probably somewhere near the bottom of the distortion on the floor, and that max value may be above 19620 Newtons, or possibly less depending on what exactly the 0.5 mm measures.

Also remember that the suggested 1mm stopping distance is shared between the weight and the floor. Probably the steel weight will deform (and spring back) more than the concrete.

Yes, the object will continue to change its center of gravity even beyond the point where the floor reaches its maximum distortion.  The stopping distance is thus not a nice clean value like 0.5mm.  It's a dynamic process.
Is the 0.5 mm a measure of max floor distortion, or is it a measure of the distance traveled by the center of gravity of the falling object from the moment of first contact to the moment of reaching the lowest point?  If the former, the max force is possibly less than 20k Newtons, and if the latter, it is definitely more.

One good point is that the force is distributed over an area, so if the contact area between your weight and the floor is say 10 square millimeters the actual floor load is 1/10 of that of a 1 millimeter area.

The change of area does not change the load (force) at all.  The area is pretty irrelevant to a computation of force and has more to do with pressure.  The question did not mention pressure (which is measured Pascals, not Newtons), even if pressure is more likely to damage a floor than is force.
Like the force computation, the max pressure computation cannot be computed by force/area.  That would only be the average pressure, not the max pressure reached presumably near the middle given a round mass hitting a flat floor.

The gym that I use has thick rubber mats in areas were weights are present, these would be a help.

That would serve to reduce both force and pressure, yes.  It reduces the former by increasing stopping distance, and it reduces the latter by increase of contact area.

I understand that given the stopping distance you can calculate the force required to decelerate the falling weight. Syphrum gave an example where the stopping distance is one mm, but surely this distance depends on the characteristics of the materials absorbing the impact?

Sure, a more spongy material will have a longer stopping distance, so a 1 Kg nerf ball dropped from a meter will exert far less force on the floor.

"how do you know the stopping distance?"

You don't.  You might be able to compute it if you were a materials engineer, but I'm not one of those.  Best to just measure it.