# Homework Help: Proton Motion

1. Oct 25, 2007

### boogiebear

1. The problem statement, all variables and given/known data
So I suck at physics, especially when it comes to going from one thing to another, in this case Voltage and PE/KE to linear motion. Anyways here's the question, any hints as to how I should go about to solve it would be much appreciated

A point charge of -10nC is at the origin. Points A and B are respectively 10 and 20 cm away from the point charge (to the right of the charge). The problem first asks for V each at pts A and B, then Delta V between points A and B, then the potential energy each at A and B. The main part of the question I'm having trouble with is as follows:

part d. Calculate the speed at point B of a proton that was moving to the right at point A with a speed of 8 x 10^5 m/s

So i know that change in voltage and change in PE is positive, and that since the point charge is negative the proton slows down as it moves toward point B. However, I'm not too sure where to go from here. So would the increase in PE translate to a decrease in KE??

2. Relevant equations
KE = .5mv^2
PE = (k * q1 * q2)/r

3. The attempt at a solution
PE at A = (8.99E9)(-10E-9)(1.6E-19)/(.1)
PE at B = (8.99E9)(-10E-9)(1.6E-19)/(.2)

(PE at B) - (PE at A) = 7.19E-17 Joules

Then I subtracted the number I found above from the initial kinetic energy of the moving proton to find KE at point B and solved for Velocity which came out to around 7.44E5 m/s but I have a feeling that this is the wrong answer