# Static Electricity an Aliens

1. Jan 30, 2009

### toasted

1. The problem statement, all variables and given/known data

Imagine that space invaders could deposit extra electrons in equal amounts on the earth and on your car, which has a mass of 1200kg. Note that the rubber tires would provide some insulation. How much charge Q would need to be placed on your car (same amount on earth) in order to levitate it (overcome gravity)? (Hint: Assume the separation distance is the radius of the Earth- and that the Earth's charge is sprea uniformly so it acts as if it were located at the earth's center.)

2. Relevant equations

Fg =Gm1m2/ d2

and possily: F= kq1q2/d2

3. The attempt at a solution

So i think you would first find the force of gravity between the earth and the car, however from there im not sure how you would determine the charge from that in order to lift the car. Could someone please help me with this part?

2. Jan 30, 2009

### Dick

Equate G*m1*m2/d^2=k*q1*q2/d^2. m1 is mass of the earth, m2 is mass of the car and the problem told you q1=q2=Q.

3. Jan 31, 2009

### toasted

I tried the technique that you said and I got the charge to be 5.3x 107C, but according to my teacher i was suppose to get 7.3 x 103C. Could you try to clarify why i got this answer? When you try it what answer do you get?

4. Jan 31, 2009

### Dick

I get the same thing your teacher got. Maybe you'd better show your work.

5. Jan 31, 2009

### Dick

I think your number is 5.3*10^7 coulombs^2=Q^2. You have to take a square root to get Q.

6. Jan 31, 2009

### toasted

O I forgot that I had q2 on top! thank you so much for your help, and explaining it through!