Physics problem help...
 

Can anybody help me out with this problem?

A unit of area often used in measuring land areas is the hectare, defined as 104 ^2. An open-pit coal mine consumes 98 hectares of land, down to a depth of 25 m, each year. What volume of earth, in cubic kilometers, is removed in this time?

As easy as it might be, I am having trouble with it... Any help would be appreciated!

Convert hectares to m^2.

Multiply that by 25 m. That will give you an answer in cubic meters

Convert that to cubic kilometers.

The one thing that your missing, and I'm guessing you didn't type it is that it asks "is removed in this time?"

It doesn't specify what "this time" is. Whatever it is, just multiply your answer by it.

No time specified, and the answer I get is 2.45E12 , but it doesn't like that answer...

Is that correct?

98 hectares = 980000 m²

980000 m² x 25 m = 24500000 m³ = 0.0245 km³

Maybe?

i hate physics
(sry had to say it, physics student myself)

Yeah, not fun... lol

Although I used up my online attempts, your answer was right... Oh well, lol. Thanks!

Seems like I remembered something from college.

Want to try once more? Ive done this problem over and over, yet nothing works out for me. I hate confusing conversions...

A mole of atoms is 6.02 x 10^23 atoms. How many moles of atoms are in a domestic cat with a mass of 8.6 kg? Take the mass of an average atom in the cat to be 9.5 u.

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I always end up getting 6.652E32 or 4E50 which doesn't seem right...

Dimensional Analysis problems are actually really easy, once you get a rhythm down the solving them. The key is to start with what you know, and on the opposite side of the page, write what you have to get to.

Then take it slow and go step by step, converting only 1 thing at a time. Don't try to get fancy, it only leads to mistakes. When you do this, actually go through and cross-off the units that you converted.

Example:

How many inches in 2 yards?

2 yards x 3 feet / 1 yard x 12 inches / foot = 36 inches

Same goes for the problem you posted:

Start with 8.6 kilograms and end with atoms...

8.6 kg x (1000g / 1kg) = 8600 grams

8600 g x (1 mol / 9.5 g) = 934.78 moles of Cat

934.78 moles of Cat x 6.02 x 10^23 atoms = # of atoms

Tada!

The answer is supposed to be in mol, not atoms, although the question sounds like it. And the cat isnt 9.5g but 9.5 u (Atomic mass units?), which is 1KG to 6.022E26 AMU...

nvm, used of my chances, haha. Thanks guys!

AMU = grams / mol

So I just took it a step too far.

Does 934.78 moles sound right?

I think they threw the Arvogravo (I can't remember what the name really is) number in there to mess with you.

I'm going to be honest, I'm not very confident in this answer and really don't want to get scratch paper out to really try it. It's my bed time. :yawn: