Some Easy Questions

[FatMike1990]

I am a high school student and I am having some problems with some questions. I was absent for a week in my Astronomy class and I haven't been able to get some one to help me at school.


1. How many atoms in your body? (Use your body weight!)
I weigh 156 pounds, would I just multiply 156 x Avogadro number

2. How many atoms in the Sun (Hint: Mass of Sun is 2 x 1030 kg).
Would I multiply mass of the sun by Avogadro number

3. The distance between the Sun and the Earth is 1.5x108 km. How long does it take for light to travel? (Express it in unit of second, and then in unit of minutes.)

[yuzuha]

1} hint. You are over 90% water, which has a molecular weight of about 18 grams per mole
2) the sun is mostly hydrogen.
3) The speed of light in a vacuum is about 3x108 meters/sec.

[01101001]

We tend not to answer homework questions directly, but we don't mind helping you think some.


1. How many atoms in your body? (Use your body weight!) I weigh 156 pounds, would I just multiply 156 x Avogadro number

Not unless Avogradro's number were the number of atoms per pound of human. As I recall, it isn't. From the combined power of my hazy memory and the fact-bloated Internet (http://en.wikipedia.org/wiki/Avogadro's_number), Avagadro's number is...


formally defined to be the number of "entities" in one mole,[1] that is the number of carbon-12 atoms in 12 grams (0.012 kg) of unbound carbon-12 in its rest-energy electronic state.

If only you were made of unbound carbon-12 and you knew your weight in grams...

Can you think of any way to get there? I mean, without turning yourself into pure carbon.


2. How many atoms in the Sun (Hint: Mass of Sun is 2 x 1030 kg). Would I multiply mass of the sun by Avogadro number

See commentary for question 1.


3. The distance between the Sun and the Earth is 1.5x108 km. How long does it take for light to travel? (Express it in unit of second, and then in unit of minutes.)

If only we knew the speed of light when traveling through space! Then we could divide the distance by the speed and get the time. Right?

Let's see... If I have to travel 30 miles and my car goes 60 miles/hour, then... distance divide by speed... 30 miles / (60 miles/hour) is 1/2 hour. That seems right.

Where can we get the speed of light? Who might know? Not my hazy memory! Perhaps the fact-bloated Internet (http://en.wikipedia.org/wiki/Speed_of_light).

[FatMike1990]

Well I am still not understanding 1 and 2 but I thing I got Question 3.

The distance between the sun and Earth is 1.5x10^8 (150,000,000 miles) and speed of light travels at a speed of 11,176,943.82 miles per min. Divide miles by minute and I get 13.42048438 minutes.

Speed of light travels at a speed of 186,282.397 miles per second, Divide miles by second and I get 805.2290631 seconds

[hhEb09'1]

The distance between the sun and Earth is 1.5x10^8 (150,000,000 miles) Careful. As you say in the OP, that is 1.5x108 km, kilometers, not miles.

[01101001]

The distance between the sun and Earth is 1.5x10^8 (150,000,000 miles)

Oops. Don't you mean 150,000,000 km? Watch the units, carefully.


and speed of light travels at a speed of 11,176,943.82 miles per min. Divide miles by minute and I get 13.42048438 minutes.

Close. Unfortunately 150 000 000 km is around 93 000 000 miles.

93 000 000 miles / 11 176 943.82 miles/minute is about 8.5 minutes. You can do it more accurately.


Speed of light travels at a speed of 186,282.397 miles per second, Divide miles by second and I get 805.2290631 seconds

93 000 000 miles / 186 282.397 miles/second

I get about 500 seconds.


Well I am still not understanding 1 and 2

You're not all carbon. But, you could estimate what you actually are mostly, as yuhuza did above, water, and we can figure out how much a mole of that weighs -- can't we?

We could then figure out about how many moles of water your body might be. That would be a useful number, because thanks to Mr. Avogadro, we know how many molecules there are per mole.

If you get all your pounds converted to grams, and figure out how many moles of water that would be, then you can compute how many molecules would make your weight if you were all water, and you almost are.

And then you need the number of atoms, not molecules. But, if you know what atoms make up a water molecule, you should be all set.

Here comes the Sun. Hydrogen mostly. What is the mass of a mole of hydrogen? How many moles of hydrogen would the Sun be? How many atoms would that be?