We can call it assault soon as someone tells me why the cop was standing directly in his path, if he was free to go... The cop stands out far enough he can't get past, sees the chair coming, and purposely places himself to initiate the incident, allowing him to knock a disabled person out of their wheel chair. If you are defending this... seriously take a step back. Cops aren't always the good guys. This is a bully bullying a disabled person because for whatever reason the disabled person pissed them off. This is assault via color law.
