Constant velocity motion

But have we solved every mysteries related with speed? And at fundamental level Speed and velocity are same thing... so how do we know when to distinguish and when to not?

Why the mass of an object increases when we try to move it to reach speed of light?

@Swayam Jha Speed is a scalar quantity and Velocity is a vector quantity. To help explain this, if a person takes a step forward and a step back to end up in the same spot, then their speed is the rate at which they are moving, however in this case because they are back at their point of origin, there velocity is 0 because their overall location has not changed...