(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A uniform electric field exists in the region between two oppositely charged parallel plates (1.64*10^{-2})m apart. A proton is released from rest at the surface of the positively charged plate and strikes the surface of the opposite plate in a time interval (1.60*10^{−6})s.

Find the speed of the proton at the moment it strikes the negatively charged plate.

2. Relevant equations

[tex]x = x_0 + v_0 t + (1/2) a t^2[/tex]

[tex]v = v_0 + a t[/tex]

[tex]v^2 = v_0^2 + 2 a (x - x_0)[/tex]

3. The attempt at a solution

Okay so I found a by using this equation ([tex]x = x_0 + v_0 t + (1/2) a t^2[/tex]) and it came out to be 1.2812*10^10.

To find velocity when it hits, I realized that I get a different answer when I use this ([tex]v = v_0 + a t[/tex]) and that ([tex]v^2 = v_0^2 + 2 a (x - x_0)[/tex]). The correct answer came out when I used the latter equation.

So my question is, why is it that I can't use the first equation? Shouldn't they come out to be the same answer? Did I do something wrong?

Thanks guys.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Confused on which equation to use (simple question)

**Physics Forums | Science Articles, Homework Help, Discussion**