what happens if ac is fed to a dc motor?
Depends. If it is what is known as a universal motor, the motor will start and run. However, most motors are optimized to run on either AC OR DC and so will only work satisfactorily on the system it is designed for.
You will cause serious damage. DC means direct current, meaning it is powered directly from the generating source (battery, transformer, inverter, etc.) AC means alternating current, meaning it changes polarity. If you have seen the wording 120v 60Hz, that indicates that the sinewave is 60Hz, meaning the polarity changes 60 times a second.
