Balanced forces - Electrical and Gravitational forces

[hhhp8cec1]

Homework Statement

Lets say I have two objects, with the same mass, m, and charge (+q), with a distance r between them. They repel each other due to electrical force, but they are attracted due to gravitational force.

My questions is: What should be their mass m such that they feel no net force?

Homework Equations

F (g) = GMm/r^2
F(e) = k q1q1/r^2

The Attempt at a Solution

F(g) = F(e)
kq^2/r^2 = Gm^2 / r^2

Im getting a really small number when solving m = sqrt (k(q^2)/ g).
Is this a trick question ?
When taking the ratio of F(e)/F(g) I see that the electrical force is much stronger than Fg.

Thank you [/B]

 

[mfb]

What do you get, and what did you use as charge?

When taking the ratio of F(e)/F(g) I see that the electrical force is much stronger than Fg.

That ratio should be 1 if your numbers are right.

 

[haruspex]

Is this a trick question ?

Regardless of the question, you had two equations and found what you believed was the solution of those equations, but on plugging the numbers back in it does not seem to be a solution after all. So you made a mistake either in obtaining the solution or in checking it.
Please post your working.

 

[lekh2003]

When taking the ratio of F(e)/F(g) I see that the electrical force is much stronger than Fg.

That's because your answer is wrong. You have not posted any of the actual work or actual numbers or your answer, so how are we supposed to see where you went wrong?

 

[Delta2]

You seem to setup and solve the equation correctly, but you must have done somewhere a mistake when replacing the quantities involved with real numbers and doing the arithmetic operations.

 

[hhhp8cec1]

Sorry for the lack of information. This is how I solved this question:

 

[haruspex]

Sorry for the lack of information. This is how I solved this question:View attachment 218311

I make it slightly more, 1.85x10^-9kg. But you have not shown the rest of your calculation, where you find that F_g is much less than F_e.

 

[hhhp8cec1]

Yes, in scientific mode I get the same result, 1.85 x 10 ^-9.

What got me confused was this explanation:
http://www.batesville.k12.in.us/physics/phynet/e&m/electrostatics/michaels_question.htm

"So, in fact, we have answered the original question - there is no distance at all at which the gravitational force between two electrons balances the electric force between them!"

But in this question, they referred to 2 protons/electrons, so that is a different case... When using the result 1.85x10^-9 and solving for the ratio I get 1, cool = ]

Thank you for your help

 

[haruspex]

What got me confused was this explanation:

You originally considered the answer 1.85x10^-9 kg as tiny, but you did not say you were dealing with electrons. In those terms, that mass is huge, over 10²¹ electron masses.