# Homework Help: Accelaration of a sphere being aimed at a point charge

1. May 9, 2012

### lukeharvey

Question

A point charge $q_{1}=15$$\mu$F is held fixed in space. From a horizontal distance of 4.00cm , a small sphere with mass $4.00\times10^{-3}kg$ and charge $q_{2}=2.00$$\mu$F is fired toward the fixed charge with an initial speed of 43.0m/s. Gravity can be neglected.

What is the acceleration of the sphere at the instant when its speed is 26.0m/s?

Attempt at the problem

I used F = (Ke X Q1 X Q2) / (r^2)

and then used Ae=Fe/Me

However this is wrong. Does anyone have any ideas which could help me? Thanks

2. May 9, 2012

### Staff: Mentor

What did you use for the distance r? That's the first step: find out where the sphere will be when it reaches the given speed. (Hint: Consider energy.)

Also: what kind of unit is μF? Did you mean μC?

3. May 9, 2012

### lukeharvey

Yeah sorry i meant μC and i used the horizontal distance given.
I take it i should use conservation of energy to find the new distance when it reaches the given speed?

4. May 9, 2012

Exactly.