# Homework Help: How much of our body is vacuum and other questions

1. Jun 30, 2006

### Lisa!

I took a physics exam last week and these questions were there?
1. How percentage of our body is vacuum?(ther's nothing there and it's totally empty)
2. Ifwe want to share the all amount of suplies of salts in the world between all of people in the world, how much salt each 1 would get?

As far as I remember and I can translate the questions were this way.(I don't remeber whether they gave us any extra info. like weight of something but I guess they didn't and they just wanted to examine our general knowledge) I even don't know what the hell they're about. I mean weyther they're chemistery or physics problems but since it was a physics exam I thought I'd better post them here.

Thanks:shy:

2. Jun 30, 2006

### wolram

I am sure the first question is a trick question, or worded wrong, the second question is equally ambiguose. the person that asked these questions is nuts.

Last edited: Jun 30, 2006
3. Jun 30, 2006

### Lisa!

Why do you think it's tricky?

Agree with you! I just don't know how on earth they expect us to solve it withought giving any info. !

4. Jun 30, 2006

no offense, but those are the stupidest exam questions i have ever seen

5. Jun 30, 2006

### J77

:rofl: :rofl: :rofl: :rofl: :rofl:

:surprised @ Questions!

6. Jun 30, 2006

### Lisa!

:rofl:
That was an entrance exam for teaching at high school. I guess they just didn't know what the hell they should write!

7. Jun 30, 2006

### sleepsleep

if i took the exam, i would answer probably like below:

1. How percentage of our body is vacuum?(ther's nothing there and it's totally empty)

need more "defination" in order to answer this question, what is nothing, empty, here and totally ?

2. Ifwe want to share the all amount of suplies of salts in the world
between all of people in the world, how much salt each 1 would get?

each one would get different kilogram of salt, if he works in politic, he would gets more :) maybe a mountain or several mountains of salt. if he is begger or somebody people wouldn't care, he gets null salt.

you mentioned "if we want to share ...", but who is we? if it is you, probably you wouldn't share it, you would sell those salts :p

:D :D

8. Jun 30, 2006

### Tojen

1) If I'm reading this correctly, it might be referring to the fact that all matter, people included, is mostly empty space. If that's what the question means, though, calling it a vaccuum is kind of misleading.

2) There must be estimations of the amount of salt in the oceans, though I don't know how youy'd do it for the other 30% of Earth. Or maybe it's referring to the amount of salt produced in the world. Either way, it doesn't seem to have anything to do with physics, so I'm probably on the wrong track altogether.

Edit: Each person gets 1/6 billionth of the world's salt.

Last edited: Jun 30, 2006
9. Jun 30, 2006

### Lisa!

The proble was that it was sorta exam that gives you 4 options to choose from and you couldn't write such things!

10. Jun 30, 2006

### nrqed

Ah! You mean it's multiple choice? So what kind of choice were given? (were they numerical?) Do you remember? Then maybe it is possible to eliminate 3 easily.

For the first question, I would personally consider the volume of a typical nucleus relative to the typical volume of an atome (about 4/3 Pi (10^-15 m)^3 relative to about 4/3 Pi (10^-10)^3 so a ratio of about 10^(-15) is "occupied" by the nuclei, which means about (10^-13) % of the volume is "not vaccum". (This is admittedly a very crude answer and is not completely meaningful given that nuclei are not "full" themselves but this is the only thing that comes to my mind using a simple calculation)

Patrick

11. Jun 30, 2006

### Lisa!

H&C forum and then intoductory physics, eh?

I told you that was a test to hire a high school teacher but the questions were beyond the knowledge of a high school student. Note that they were examining people with Bsc or MSc in Physics, So I don't think that would be the right place sinc I'm not even a student. Anyway let's forget where we are...

Ya!
For the 1st question I guess:
1. 25%
2. 70%
3. 55-60%
4. 99%
Thanks! Glad that finally someone tried to solve the problem.

12. Jun 30, 2006

### Kurdt

Staff Emeritus
I agree with the analysis of the first question with the atomic and nuclear radius but I have no idea about the second. Seems a bit strange but perhaps if you post the choices for that aswell we can wok it out.

13. Jun 30, 2006

### J77

Is most of the salt in the world contained in the oceans?

14. Jun 30, 2006

### nrqed

The other problem is one of those order of magnitude calculations based on educated guesses (there are a few famous ones. Enrico Fermi is said to have been a masster at those (well, he was brilliant at many things)).

It would help if I would see the possible choices but I will try a wild guess and see what I get (the answer could differ from mine by a factor of 10 or even 100 but hopefully the possible answers are so widely different that it should be clear which is right.

I will estimate the amount of salt in the oceans. I will use that water has a density of 1000 kg/m^3, the radius of the Earth is 6300km and that 70% of the surface is covered with water (those are not exact numbers! I am just using rounded values that I knew by Earth, not values that I looked up somewhere. The density of salt water is surely different than exactly 1000 kg/m^3 for example). Now, a guess I have to make is the proportion of salt in ocean water. That's an educated guess. I guessed that 1% of the mass is salt water (I looked it up afterward and it turns out that it's closer tp 3.5% so my answer will be 3.5 times too small for this guess alone). Next thing is that I thought that I would use an average depth of 1km for the ocenas (the real number is surely within a factor of 2 or 3 of that number).

So total mass of salt = $$4 \pi \times 0.7 \times 1000 m \times ( 6.3 \times 10^6 m)^2 \times 1000 {kg_{water} \over m^3} \times 0.01 {kg_{salt} \over kg_{water}}=3.5 \times 10^{18} kg_{salt}$$

The next problem is to estimate the mass of one grain of salt. It is much less than 1 gram. This is the point that I felt the less sure about. I thought that I would use 1 microgram. Since I wasn't sure, I thought about it a bit more. The density of salt (a solid, but not very dense of a solid) is probably of the order of the density of water, 1gr/cm^3. How big is a grain of salt? I tried to picture one and decided that treating it as a a cube of half a millimeter of side was a good approximation. This gives a mass of $1.25 \times 10^{-4} gr$ instead of 10^(-3) gr as my first guess. This showed that my initial guess was not totally crazy but I decided to use 10^(-4) gr (by the way, treating the grain of salt as a sphere of dimater 0.5 mm just changes the answer by a factor of 0.52 ).

If I use this, I then get $3.5 \times 10^{25}$ grains of salt which, for 7 billion people means $5 \times 10^{15}$ grains of salt per person.

That's my wild guess!

Patrick

15. Jun 30, 2006

### Lisa!

Unfortunately I don't remeber the choices for the 2nd 1. I guess 1 of them was $$10^21$$. The nrqed's answer seems close to the options so it could be right!