TV signals (PAL and NTSC) were 50 and 60 Hz so as to be in sync with the flickering of electric lamps.

When film is converted to 50 Hz TV, the film is sped up 24->25 fps and every frame shown twice. When converted to 60 Hz TV, there is "2:3 pulldown": every even frame is shown twice, every odd thrice. (Actually, both PAL and NTSC have interlaced video modes, with only every other line updated each frame, so as to conserve bandwidth.)

BTW, when 60 Hz computer monitors were introduced in Europe and used in office spaces with fluorescent lights with passive ballasts that flickered at 50 Hz, some sensitive users suffered headaches from using the computer screen for too long. These days, both fluorescent lights and LCD backlights tend to flicker at much higher frequencies that it isn't much of a problem.