This is a multiple choice question that, after one incorrect attempt, I got correct; however, I want to actually understand what the explanation means. I'm hoping someone here can help. 1. The problem statement, all variables and given/known data Two electrons, each with mass m and charge q, are released from positions very far from each other. With respect to a certain reference frame, electron A has initial nonzero speed v toward electron B in the positive x direction, and electron B has initial speed 3v toward electron A in the negative x direction. The electrons move directly toward each other along the x axis (very hard to do with real electrons). As the electrons approach each other, they slow due to their electric repulsion. This repulsion eventually pushes them away from each other. Which of the following statements about the motion of the electrons in the given reference frame will be true at the instant the two electrons reach their minimum separation? (see #2 for more on this) 2. The attempt at a solution The answer to this was "Both electrons are moving at the same (nonzero) speed in the same direction." (I chose "Both electrons are momentarily stationary" initially). MASTERINGPHYSICS SITE FEEDBACK: If at a given moment the electrons are still moving toward each other, then they will be closer in the next instant. If at a given moment the electrons are moving away from each other, then they were closer in the previous instant. The electrons will be traveling in the same direction at the same speed at the moment they reach their minimum separation. Only in a reference frame in which the total momentum is zero (the center of momentum frame) would the electrons be stationary at their minimum separation. This is the aspect I'm looking for help on; it talks about reference frames and how the difference in which reference frame I'm using will change the answer here. Perhaps I'm confused by the concept of the reference frame to begin with, but can someone break this down for me? Also, when the question says "in the same direction" do they mean "both moving towards each other" or "both in either the +x or -x direction"? Thanks for any help; I want to actually understand this, not just get the right answer. (I know this isn't the usual format for these questions but this is an online course with a very unresponsive professor).