Ok just another way to explain things in a very simplified way.

A lens has a focal length that is the distance from the focal point of the lens to the surface of the sensor so a 50mm lens is 50mm from the sensor surface and a 100mm lens 100mm from the sensor surface, as the image circle on the sensor plane stays the same diameter then this determines the angle of view.

As you can see the full frame sensor captures all the image projected but the crop sensor (hence its name) only sees the central area, this means that a crop sensor camera has a field of view effectively of a longer focal length lens BUT the perspective will be different.

So a 50mm lens is always 50mm its the sensor that determines how much of the projected image it can save.