# Homework Help: Confused on which equation to use (simple question)

1. Mar 8, 2009

### KYUI

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

3. 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.

Last edited: Mar 8, 2009
2. Mar 8, 2009

### hage567

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

3. Mar 8, 2009

### 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. :)

4. Mar 8, 2009

### hage567

lol you're welcome.