1. Jan 24, 2005

### CinderBlockFist

What exactly is meant by earth radii?

In my hw problem, it is asking, "How many Earth radii must the same object be from the center of Earth if it is to weigh the same as it does on the Moon?"

Well, I found out how far the distance the object has to be from the earth, but is this the same as Earth radii? If it is not what exactly is it asking for?

2. Jan 24, 2005

### christinono

I would think it is that distance (from object to earth's center) divided by the radius of the earth, which is, I think, 6.38 x10^6 m, or something like that.

3. Jan 24, 2005

### CinderBlockFist

ok thanks leme try it out, see if it is the correct answer, if not, It's all your fault! jk man, thanks for the quick response. brb.

4. Jan 24, 2005

### CinderBlockFist

crap its wrong =\

5. Jan 24, 2005

### Cantari

6. Jan 24, 2005

### christinono

Maybe you're doing the first part of the problem wrong. Show your work and I'll see if I can help...

7. Jan 24, 2005

### CinderBlockFist

The mass of the object is 7.645 kg. The object has to weigh 12.499 N.

So what i did was plugged those two variables into w = mg

and i get 12.499 N = (7.645 kg) x g

so i got gravitational acc. has to be 1.634 m/s^2

Now I pluged it into the formula gravitational acc. = (Gravitational constant)(Mass of earth)/r^2 to find the radius distance of the object.

i got r = 1.5623 x 10^ 7 m.

so i added r to radius of earth to get 2.1993 x 10^7 m, then divided by the radius of earth to get 3.45 earth radii, which was incorrect. Where did i go wrong?

8. Jan 24, 2005

### christinono

Where did you get the 12.499 N from? Was it given in the question?

9. Jan 24, 2005

### CinderBlockFist

yea thats given

10. Jan 24, 2005

### CinderBlockFist

maybe i wasnt supposed to add the earth's radius in the last part,is it already included for my value for r?

11. Jan 24, 2005

### christinono

I got 6.56 x 10^16 m. Divided by the radius of the earth, i get 1.03 x 10^10.

12. Jan 24, 2005

### christinono

Sorry, i used the wrong constant. I used the k constant instead of the G constant. let me try it again...

13. Jan 24, 2005

### CinderBlockFist

are you adding the radius of the earth, to the value of r that was in the denominator? or just leaving r , as is?

14. Jan 24, 2005

### christinono

OK. I got the same r value: 1.5623 x 10^ 7 m.
But I'm pretty sure that's the TOTAL radius, meaning it's the distance from the center of the Earth. In this case, you don't have to add the radius of the earth to that r value. You just divide r by the radius of the earth. I got
2.45.

15. Jan 24, 2005

### CinderBlockFist

Oh crap i got it! i wasnt supposed to add the earths radius. THe correcta nswer is 2.45. THanks chris, u da man!

16. Jan 24, 2005

no problem