Calculating Charge and Acceleration of Two Charged Spheres

[Josh930]

Homework Statement

Two very small 8.55-g spheres, 20.0cm apart from center to center, are charged by adding equal numbers of electrons to each of them.

Disregarding all other forces, how many electrons would you have to add to each sphere so that the two spheres will accelerate at 15.0 g when released?

Homework Equations

F=ma
F=k(q1q2/r^2)
q=ne

The Attempt at a Solution

for one.. i don't know why the acceleration is at 15.0 g?

But i did try to set the forces equal and solve q, then plugged q into the equation q=ne and it wasn't right.

Please help

 

[rl.bhat]

Show your calculations.

 

[Josh930]

F=ma=F=k(q1q2/r^2)

q1=q2; let Q^2=q1q2

sqrt(mar^2/k)=Q

sqrt((8.55g*15.0m/s)((2m)^2)/k)=Q

Q=2.39e-4

Q=ne

n=2.39e-4/1.6e-19

=1.49e15 electrons

 

[rl.bhat]

The acceleration is 15*g i.e. 15*9.8 m/s^2
In the solution you have written (2m)^2. What is that?

 

[Josh930]

they are 20cm apart? so .2m

 

[Josh930]

i messed up and wrote 2m. so 15g is 15x the acceleration of gravity

 

[rl.bhat]

Yes.