# Homework Help: Colliding Particles using energy

1. Jun 19, 2010

### seto6

this question is from my previous assignment which was due 2 weeks back. just revising for mid-term not sure how to solve it. if someone explains it thoroughly it would be grate

1. The problem statement, all variables and given/known data
An alpha particle is a nucleus of Helium. It has twice the charge and four times the mass of the proton.

A proton and an alpha particle headed directly toward each other, had each initial speed of 3.9×10−3 c when they were far away.

Here, as is customary when describing processes involving nuclear targets, the speed is expressed as a fraction of c, the speed of light.

What is the distance of closest approach between the proton and the alpha particle?

Hint: There are two conserved quantities. Make use of both.

2. Relevant equations
U=Eqd or KqQ/r
K=1/2*(mv^2)

3. The attempt at a solution

.5m(v_i)^2 + Eq(d_i)= .5m(v_f)^2 + Eq(d_f)

where (V_f)=0 then i solved for (d_f)
and got the 1.87*10^-27 but the answer should be d_f=1.3*10^-13

could some 1 explain this to me and the reasoning!! please

Last edited: Jun 19, 2010
2. Jun 19, 2010

### Staff: Mentor

You are assuming that V_f = 0. Make use of the hint that you were given.

3. Jun 19, 2010

### seto6

this question is from my previous assignment which was due 2 weeks back. just revising for mid-term not sure how to solve it. if someone explains it thoroughly it would be grate

4. Jun 19, 2010

### Staff: Mentor

Why don't you make an attempt using the hints I just gave?

5. Jun 19, 2010

### seto6

v_f is the final velocity , at minimum distance it can reach it will be zero.no?

6. Jun 19, 2010

### Staff: Mentor

No it won't. Use the hint that the problem gave you! What else is conserved besides energy?