Is backspin not created when the club initially impacts the ball?
Yes, and there are a number of different elements that go into determining what this is. The cleanness of the contact, and how hard you hit it for example. You could hit 10 8I at 122% and get 10 different results due to the quality of the shot (in 3 click determined by the snap) So before you even start looking at the physics you are not even sure you are comparing like with like. The snap goes to 5 decimal places and once you dont hit a perfect snap the rules are changing and the vlaues are different.
Are backspin and gravity supposed to be interchangeable? Can gravity really be substituted with backspin?
I have no idea what this means, one is a force that acts in the vertical direction, the other is a state that exists on the ball, the two forces act in completely different ways and are not interchangeable
Is gravity supposed to create backspin? Are backspin and roll the same thing? Do they look the same?
The only thing that creates backspin is the shot impact conditions. At that point the amount of backspin is set. During the course of the ball flight spin decay will reduce the amount of backspin and this effect is dependent on the wind too. During impact with the ground the backspin will be reduced by the contact of the ball with the ground. The number of bounces, will determine how much spin is lost during bouncing and the relative values of horizontal velocity and backspin will determine how much backwards velocity the ball will have once it transitions into the roll state.
Is the gradient of a green meant to increase the spin rate of a golf ball?
No and this question implies you dont understand how it works at all. The gradient of the green at impact changes the angle at which the ball bounces. This angle change in turn affects the relative values of vertical velocity and horizontal velocity (Restitution affects the total velocity of the ball). A ball with significantly less horizontal relative velocity will have, well, less horizontal velocity for the backspin to overcome during the bounce state. Finally at the point at which the ball transitions from bounce to roll the simple question is.... Is the amount of backspin higher that the remaining forward horizontal velocity of the ball such that the ball will spin backwards.
In addition to this the steeper the ball impacts with the ground, the less horizontal skidding occurs between the ball and the ground, resulting in the amount of spin taken off the ball being slightly less than when the ball impacts with a higher horizontal velocity component.
Finally a ball landing into an upslope and spinning backwards will now be running downhill exaggerating the effect of the backspin.
So there are a multitide of different ways small changes in the slope of the green can have on the relative values that go into determining the amount a ball may spin back or forward on any particular shot.
Can the gradient of a green cause a ball that has 3 yards of forward momentum to violently spin 12 yards backwards?
Since gradient of a green has no ability to create backspin this is a somewhat specious question. The question really should be Can the gradient of a green cause the bounce of a ball to be changed such that the resulting effect on the ball yield a change in the relative values of spin, bounce angle, and horizontal and vertical velocity components and the answer to that based on the data is yes.
One final thought... if what you are looking for is a mathematical equation to apply to a shot you will not find it here. Just like golf I have seen tour pros hit shots that land by a pin and zip off due to backspin 30 feet away. Can we assume that their plan all along was to spin it back 30 feet away from the hole, I doubt it. They just either got a lot more spin than they intended or they hit into an upslope or they did not hit the ball as far as they intended. So why does your 1 in 10 8 iron spin back? It could be because:
1) you hit a particularly pure shot resulting in higher spin at launch,
2) you were hitting off an upslope which changed the launch angle of the ball causing the impact angle with the green to be steeper and the resulting relative horizontal / vertical velocities to change and or the relative spin loss on imact to change
3) your ball landed on an upslope in the green causing the ball to bounce at a more vertical angle resulting in a change in the relative horizontal / vertical velocities meaning the same amount of spin will spin back further as there is less horizintal velocity to overcome.
4) Your shot was into the wind
5) You adjusted the Launch Angle Center which changes everything
Can the physics result in some strange results from time to time, yes sure it can, you are writing a set of code that allows the real world physics to make a determination of what the ball is going to do when there are an infinite number of possible variables. This is part of what I like about it. Who has played a round of golf and said how on earth did that happen when looking at a bounce or a result. Most people I guess. Our Physics allows these things to happen, it is not pre ordained and not programmed in. Is it perfect no, and we will continue to tweak it as we have time to look at some of these things. But your comment that the whole game physics are broken bacause you cant hit a massive overswing 8I and get an identical result everytime is never going to get any traction.
