This is with regard to 2 boxes of different masses that are stacked on top of eachother and pushed at a constant velocity across a table that has friction. One of the sentences is right: "When the boxes move at a constant speed, there is no change in momentum. This means that the boxes are an isolated system." My initial thought was that the mistake was in saying "constant speed" when it should be "constant velocity". But is there a problem in how the system is defined as only the two boxes? Should it include the table, for example? EDIT: Also, could someone clear up exactly what the threshold for "isolated" is? This always confused me because nothing is isolated unless the system is literally everything.