# Electric potential and conservations of energy

1. Jan 28, 2012

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

For the following image V1=67.9 V, V2=52.8 V, and V3=37.7 V. A proton's speed as it passes point A is 47566.0 m/s and it follows the trajectory shown. What is the proton's speed at point B?

http://capa.physics.mcmaster.ca/figures/kn/Graph29/kn-pic2944.png [Broken]

2. Relevant equations

3. The attempt at a solution

1.ΔU=qΔV
2.ΔU= 0.5mV2^2-0.5mV1^2

i then solved for V2 and i didn't get the answer.... can someone elaborate?

Last edited by a moderator: May 5, 2017
2. Jan 28, 2012

Kk i got the answer only because i dropped the negative when i found the potential difference. can someone explain why?

3. Jan 28, 2012

### Mattszo

ΔU = -q∫$\bar{E}$$\cdot$d$\bar{r}$

Hope that helps.

You're going to have to be more specific as to what you did. I can't see your calculations, but it probably was a sign error.

Matt