Solving for Distance of Closest Approach for Moving Protons

[silver_gry]

Homework Statement

Two protons are moving directly toward one another. When they are very far apart, their initial speeds are 3.00*10^6 m/s. What is the distance of closest approach?

Homework Equations

The Attempt at a Solution

I really need help in question. I need to know how to start this question. Please help! Thank you!

 

[LowlyPion]

They both have kinetic energy.

If they have identical speeds, then they will both momentarily come to rest at their closest approach.

If they are at rest relative to each other then what is their potential energy with respect to each other at that moment?