I agree with those who said your father has been abusing your mother for a long time, and this is prolly not the first time he hit her hard, just the first time she told u about it. I hate guys that beat on women, specially in front of children. If you are in a position to stand up to him, you should do it. Let him no in no uncertain terms you will NOT tolerate any further abuse from him to your mother. If you are uncomfortable with that and or are not in a position to stand up to him because he may simply be stronger then don't challenge him in a physical way where he feels threatened, but let him know you are aware of this and you are concerned for your mother and will be watching out for her and if you have to will report him. If you feel uncomfortable with any of this then don't say any thing but be there for your mother any way you can with support and guidance! Good luck!