More on Spanking

Is all "violence" created equal? Are we really unable to make distinctions amongst different contexts and uses of force? That is the question this reader poses:

I wonder ... Suppose that a child, say, hits his sister, and the parent responds by spanking him. If the parent is convicted under this law and ordered to report to prison, and the parent refuses to surrender to authorities, would the police use physical force? If not, then why should anyone obey the law? If they will, then we are being told that physical force as a punishment is immoral ... unless it is used as a punishment for someone who used physical force ... even if that person used physical force to punish someone for using physical force.