Confused on which equation to use (simple question)

[KYUI]

Homework Statement

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.

Homework Equations

$$x = x_0 + v_0 t + (1/2) a t^2$$
$$v = v_0 + a t$$
$$v^2 = v_0^2 + 2 a (x - x_0)$$

The Attempt at a Solution

Okay so I found a by using this equation ($$x = x_0 + v_0 t + (1/2) a t^2$$) 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 ($$v = v_0 + a t$$) and that ($$v^2 = v_0^2 + 2 a (x - x_0)$$). 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.

 

[hage567]

Are you sure you just aren't making a mistake in the calculation? I get the same answer both ways.

 

[KYUI]

oh i feel stupid. thank you very much haha i calculated it couple times before i posted it but I guess I had a mistake in it.

Man, it took me awhile to write this too.

anyways, thanks. :)

 

[hage567]

lol you're welcome.