A bar walks into a man

A bar walks into a man. Oops, wrong frame of reference.

This might be a physics joke, it really isn't clear.

The joke is that normally you view a walking man as moving relative to the stationary things around him, however it is completely reasonable to view things from the point of view of any object, so if your origin of reference is the walking man, it could appear that the bar was the thing moving into the man.

It is not really that funny.