# Gravitational force and electrical force

#### leolaw

Suppose that electrical attraction, rather than gravity, were responsible for holding the MOon in orbit around the Earth. If equal and opposite charges Q were placed on the Earth and the Moon, what should be the value of Q to maintain the present orbit? Given these data: Moon = $$7.35 \ast 10^{22} kg$$, Earth = $$5.98 \ast 10^{24} kg$$ , radius of orbit = $$3.84 \ast 10^8 m$$.

This is how I approach the problem:
I set
$$G \frac{m_{1}m_{2}}{r^2} = k \frac{Q^2}{r^2}$$
and then solve for Q

Am I doing the right thing?

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#### dextercioby

Homework Helper
Nope,u should totally disregard gravity.Wiped out.Substituted with electric force...

HINT:What is the period of Moon's revolution around Earth ??

Daniel.

F

#### freemind

Hmmm, I don't see anything wrong with your method leolaw.

#### dextercioby

Homework Helper
Think at this way:there's suddenly no gravity and no Newton's law,just Coulomb's one...

Another clue:do you see somthing rather suspicious about his formula??

Daniel.

#### leolaw

dextercioby said:
Nope,u should totally disregard gravity.Wiped out.Substituted with electric force...

HINT:What is the period of Moon's revolution around Earth ??

Daniel.
Why do you need to know the period? I should use kepler's law?

#### dextercioby

Homework Helper
Not that version built with Newton's law... You can build a new one,just for the fun of it.However,you won't need the new one exactly to solve your problem...

Just apply the second law of dynamics...

Daniel.

#### leolaw

dextercioby said:
Not that version built with Newton's law... You can build a new one,just for the fun of it.However,you won't need the new one exactly to solve your problem...

Just apply the second law of dynamics...

Daniel.
Second law of dynamics mean Netwon Second law? But i thought you said I should forget about newton's law before

#### dextercioby

Homework Helper
Sorry,i mislead you... I was referring to the Law of Gravity (of Universal Attraction).

So what's the answer & how did you get it...?

Daniel.

F

#### freemind

phew, thats a relief. I was just about to check myself into a mental institution!

#### leolaw

dextercioby said:
Sorry,i mislead you... I was referring to the Law of Gravity (of Universal Attraction).

So what's the answer & how did you get it...?

Daniel.
hahaa, this problem is even number on my textbook, so i dont have the answer.
But i do think that i need another way to approach this problem because i should aviod the use of law of gravity.

And by the way, why did you say there is smoething wrong with my equation before:
dextercioby said:
Think at this way:there's suddenly no gravity and no Newton's law,just Coulomb's one...

Another clue:do you see somthing rather suspicious about his formula??

Daniel.

#### dextercioby

Homework Helper
I didn't say "wrong",i said "suspicious"... Well,by your formula,the "r" wouldn't really matter,because it would simplify,right...??

Daniel.

#### leolaw

Yes, but now i have to make sure if my idea is right

#### dextercioby

Homework Helper
What idea??Don't keep it a secret... :tongue2:

Daniel.

#### leolaw

if $$G \frac{m_{1}m_{2}}{r^2} = k \frac{Q^2}{r^2}$$ will get me the answer !

#### dextercioby

Homework Helper
Incidentally,yes...It will,though i initially advised to disregard Newton's gravity law and use the centripetal form of acceleration.

Anyway,i guess you got it solved...One way or another...

Daniel.

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