# Homework Help: The electric potential difference created by point charges.

1. Jul 19, 2009

### xkelleh

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

A charge of -3.00 C is ﬁxed in place. From a horizontal
distance of 0.0450 m, a particle of mass 7.20x10^-3 kg and charge
-8.00 C is ﬁred with an initial speed of 65.0 m/s directly toward the
ﬁxed charge. How far does the particle travel before its speed is zero?

2. Relevant equations

My question is where do I go from KE or really how would I start this problem off?

3. The attempt at a solution

I thought that it should start off with KE so I put 1/2mv^2

2. Jul 19, 2009

### Ifedayo

You don't need kinetic energy.

First you need to find the rate of acceleration (deceleration, in this case)

Force = mass*acceleration = (charge)*electron field.

Solve for acceleration.

Then use a kinematics equation to find the time.

V(final) = V(initial)*time + 1/2 (acceleration)(time squared)

3. Jul 19, 2009

### Redbelly98

Staff Emeritus
Welcome to Physics Forums, xkelleh and Ifedayo.

The acceleration is not constant, so that won't work. Please watch how much help you give in the future, and let the OP do most of the thinking.

Conservation of energy is the key here. xkelleh, you could start by writing an expression for the initial total energy.