G'day.
My name is Joe, the other day i brought a second hand starter motor from the wreckers. when i placed it in my car and turned the car on i got this grinding noise. as i needed to get somewhere i ignored the noise but then it made a loud whirring noise and the car wont start. pulled out the starter motor and half the gear that cranks the flywheel was grind-ed away :eek2:. would this be a solenoid/starter motor problem or flywheel problem?

as i needed to get somewhere i ignored the noise but then it made a loud whirring noise

I gather by this you drove the car?? If so sounds like the starter jammed in place and or not quite disengaged and stripped the teeth.
Did the wreckers test it for you before you bought it off them??
Can you see the teeth on the ring gear on the flex plate or flywheel to see what condition they are in?? They may be damaged now as well Vee