1. The problem statement, all variables and given/known data Good evening phsyicsforum. I am having a lot of trouble finding out what must be true for me to use conservation of momentum or energy (sometimes both) in problems involving electricity. Below I will post 2 different questions and what they conservation they use. 1. Find the initial velocity of an alpha particle with a mass of 6.64 x 10^-27 kg and a charge of +3.2 x 10^-19 C, if it undergoes a head-on "collision" with a gold nucleus. You may assume the gold nucleus does not move at all during the interaction. The charge on the gold nucleus is +2.53 x 10^-17 C and the distance of closest approach between the two is 4.7 x 10^-15 2. An alpha particle moving at 3.0 x 10^6 ms [east] (m2= 6.64 x 10^-27 kg and q2= +3.2 x 10^-19 C) is headed directly towards a proton moving at 5.0 x 10^6 m/s [west] (m1= 1.67 x 10^-27 kg, q1 = 1.6 x 10^-19 C). Find the distance of closest approach assuming that they start from a very far apart position. 2. Relevant equations Pto=Ptf Eto=Etf 3. The attempt at a solution Now I don't want any help answering the question itself because that is quite easy. I am only having trouble deciding when it is true to use conservation of momentum or energy or both. Do I only use conservation of momentum when 2 objects are moving? And is conservation of energy used when only one object is moving and 1 is at rest?