Need help with electric force and velocity.

[warfreak131]

Homework Statement

A 14.5 cm-radius thin ring carries a uniformly distributed 13.6 uC\ charge. A small 6.1 g sphere with a charge of 5.0 uC is placed exactly at the center of the ring and given a very small push so it moves along the ring axis (+ x axis).

How fast will the sphere be moving when it is 1.9m from the center of the ring (ignore gravity)?

Homework Equations

F=ma
F=k * Q1Q2/r^2
vf^2 = vi^2 + 2ad

The Attempt at a Solution

I set the two force equations equal to each other, and found an expression for the acceleration using .145m as r, and plugged that into the third equation and got an answer of 134. Needless to say, this wasn't the correct answer.

 

[xcvxcvvc]

Homework Statement

A 14.5 cm-radius thin ring carries a uniformly distributed 13.6 uC\ charge. A small 6.1 g sphere with a charge of 5.0 uC is placed exactly at the center of the ring and given a very small push so it moves along the ring axis (+ x axis).

How fast will the sphere be moving when it is 1.9m from the center of the ring (ignore gravity)?

Homework Equations

F=ma
F=k * Q1Q2/r^2
vf^2 = vi^2 + 2ad

The Attempt at a Solution

I set the two force equations equal to each other, and found an expression for the acceleration using .145m as r, and plugged that into the third equation and got an answer of 134. Needless to say, this wasn't the correct answer.

I'm not sure about this problem. First off, where is the ring relative to the x axis(does the x-axis shoot through the center of the ring, or does it go toward the rim, or something else?). Second, I have the assumption that the E field is not constant, so while the charge moves, the acceleration will change. That would make this problem terribly complex, though.

Perhaps you should recheck your calculation using the constant E model with energy:
$$\frac{1}{2}mv^2=Ed$$
where E is the electric field and d is the distance traveled.

 

[warfreak131]

since F=Eq, does F = coulombs law? and i assume q = the charge of the small sphere?

what about acceleration?