# Conservation of Energy question :cry:

1. May 10, 2007

### rootX

1. The problem statement, all variables and given/known data
A proton 2nm away from an oxygen nucleus is launched towards the nucleus with a velocity of 3.2E4 m/s. What is the closest approach?

2. Relevant equations
# of protons in oxy = 8
Ek =.5 mv^2
U = (kqQ)/(r)

3. The attempt at a solution
I know the answer to this question is 0.57 nm, and I also got this answer when I did this question very first time (that was like last week) however, now whenever I try this question I get 1.04 nm.

I am using this equation
change in Ek = work

-0.5mv^2 = - (9E9 x 1.6E-19^2 x 8) (1/Rf - 1/Ri)

0.5(1.67E-27)(3.2E4)^2/(9E9 x 1.6E-19^2 x 8) + 1/2nm = 1/Rf
Rf = 1.04E-9

I know I am making some mistake, but just cannot figure it out

2. May 10, 2007

### DAKONG

$$E_{k} = qV = q\frac{KQ}{r}$$
find r
q = proton (p) (move)
Q = charge against proton (8p)

in work or energy calculation not use negative

Last edited: May 10, 2007
3. May 11, 2007

### rootX

after spending like more than 2 hours, I figured out that when I was copying this question from the black board, I copied the value of v wrong, and I didn't make correction in my notes though wrote the correct answer.
So, that screwed up everthing.

Anyways, thanks a lot.