Gravity Question (2 Viewers)

[Aerath]

That's right, but the Earth's radius isn't given on the Physics data sheet so you would have to also work that out:

9.8 = (6.67x10^-11)(6.0x10^24)/r^2
r = sqrt((6.67x10^-11)(6.0x10^24)/9.8)
= 6 390 362. 642 m
= approx. 6.39 x 10^6 m

I just noticed this - does this mean that we have to derive the radius of the Earth everytime? Or can we just memorise that it's about 6380km?

 

[ratcher0071]

yes it is wrong.
y did u do the mass of the earth - the previously calculated 'r'(6.something x 10^7).
u should have done the total distance - radius of earth.

Ratcher
your maths is ok except the last formula of yours which sez r = 5.98 x 10^24 - 6.38 x 10^6. You put the wrong figures in, yu put the earth's mass rather than distance.

should have been r (from Earth's surface) = your figure of 6.38 x 10^7 - 6.38 x 10^6 = 5.74 x 10^7 m from the Earth's surface.

Incidentally if you simply divide one by the other (ratio) you get 10 x which is how far the Astronaut was from earth expressed as a ratio of Earth's radius.

Oh yeah. I see. I've edited it now, so it should be right. 0.0

I just noticed this - does this mean that we have to derive the radius of the Earth everytime? Or can we just memorise that it's about 6380km?

I think you can just memorise. But if you can't then you should derive.

 

[Continuum]

You have no life if you spent the past half an hour doing subs and sups.

I don't have the same Mathematical talent as you, so I would probably just use direct substitution, however, towards the end of your explanation, I started to make some sense of it, and the answer is pretty much the same. Nice work Jonathan.

Haha, nah it was mainly copy and paste.
I'd die if I had to type all those tags.

 

[ratcher0071]

Haha, nah it was mainly copy and paste.

That's the same thing I do. I just read that sups and subs help other people understand what you're on about. (In the FAQ's )

 

[ubermale]

I just noticed this - does this mean that we have to derive the radius of the Earth everytime? Or can we just memorise that it's about 6380km?

Well my Chemistry and Physics teachers always stress that you can only assume the values given in the question and the data sheet. My chem teacher even said that you can lose marks for stating the specific heat capacity of water as 4.18 J/g/K instead of 4.18x10^3 J/kg/K like it says on the data sheet (NOTE: you can still use 4.18 J/g/K as long as you first write down 4.18x10^3 J/kg/K), so I would always choose to derive any values that don't appear on the data sheet or the question. However, I'll still ask my Physics teacher when school starts to know for sure.