Before we get into the nitty-gritty of what happens when golf balls strike one another, let’s take a moment to appreciate that not all breaks in golf are bad. Just ask Louis Oosthuizen about his hole-in-one at the 2016 Masters. His tee shot caromed off J.B. Holmes’ ball on the green and that put it on a path to fall into the cup:
The Rules of Golf clearly addresses what to do if you hit a shot from a green and it hits another ball on the green. Rule 11.1 says to play your ball as it lies. Or, in Oosthuizen’s situation, go and retrieve it from the bottom of hole. There is no penalty when this happens. (As for the ball that was hit and almost certainly moved on the green, that ball should be replaced to where it lay before your ball hit it, with no penalty.)
However, if your ball is on the putting green and you accidentally hit another ball that was on the green before you played your stroke, it’s a two-shot penalty. You also have to play the ball as it now lies.
One thing to keep in mind is that a ball is considered on the putting green when even the smallest piece of it is touching the surface. It’s also considered on the green when it comes to rest on top of something like a leaf and is inside the edge of the green (Rule 13.1). Also keep in mind that it’s permissible to mark and lift a ball on the putting green to avoid another ball striking it. You can even do this if the other ball is in motion (Rule 11.3).
And, to be clear, if you hit a putt and then someone from off the green plays onto the green and the balls collide, there is no penalty. The ball you strike has to be on the green before your stroke is made. (Sometimes golfers unknowingly play shots at the same time.)
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