For example, "A train of mass 45 kg is traveling at a speed of 67 m/s", means that you can call the variable m the mass, v the speed, and they have particular values as part of the given in...For example, "A train of mass 45 kg is traveling at a speed of 67 m/s", means that you can call the variable m the mass, v the speed, and they have particular values as part of the given information. Sometimes, this is simply picking an equation to use ("this problem is asking us to find the center of mass, so I'm going to use the center of mass formula..."), but more often this step means picking one of the physical laws from the list above to use to solve the problem.
