The charge q4 is now replaced by charge q5 which has the same magnitude, but opposite sign from q4 (i. e., q5 = 0.1 μC). What is the new value for the potential energy of the system?

You are traveling in a car going at a constant speed of 100km/h down a long, straight highway. you pass another car going in the same direction which is traveling at a constant speed of 80km/h. as measured from your car's reference frame this other car is traveling at −20km/h. what is the acceleration of your car as measured from the other car's reference frame? what is the acceleration of the other car as measured from your car's reference frame?