Electrostatic and Gravitational Forces

[grscott_2000]

*Thanks for the help*

 

[Doc Al]

This has been driving me nuts! If a distance were specified I'd have it in an instant. Everything I've learned up to now has involved a separation between points.

Hint: Call the distance "D". Maybe you don't need the actual distance.

I know that Fgrav = -G * Mm/r^2

That's the one.

And that Fgrav = mg

No, that's the weight of the particles--their attraction to the earth, not to each other.

 

[grscott_2000]

*thanks for the help*

 

[Doc Al]

If I do a bit of rearranging and combining, would I get

Fel/Fgrav = (k q Q) / (-G m M)

Just set Fel = Fgrav.
Get rid of that minus sign--you are comparing magnitudes.

This at least cancels out the distance. I feel like this is a step closer though. Maybe my idea is correct even if the algebra is a bit off?

You are almost there. Clean up the q & Q, m & M stuff: the charges are equal; the masses are equal (and given). Solve for the charge then plug in the numbers.

 

[grscott_2000]

*thanks for the help*

 

[Doc Al]

Looks good to me.

 

[grscott_2000]

Many thanks for your help... All I often need is a prod in the right direction