Hi Friends , I had a discussion with a funny person last night. He said the following argument : Suppose there are 21 objects including the one object under consideration. Each of the 21 objects move with a velocity. Since there are 21 objects, for the object we are considering has 21 velocities with respect to 21 reference frames. We are fixing the reference frame as each object. Now since mass = (Force x Time ) / Velocity. There are 21 different mass for the same object for 21 reference frame !!! I argued him saying that mass is a constant and it cannot change. But he proved me there can be 21 different masses for 21 reference frames. How can this be possible ? ? ? Any replies will be appreciated.