How to Hit a Golf Ball Just Right - Pt1: The Ball in Flight
Hitting the golf ball just right is the #1 key to the entire game of golf (click here to read why). The key isn't the swing, although your swing plays a huge role in hitting the golf ball correctly. The key isn't lining up your stance, although lining up your stance plays a huge role in swinging effectively. The key is putting the ball exactly where you want it to go. However, you only directly control the ball for as long as it touches the clubface (usually less than half of a millisecond); the rest of the time belongs to the air. Let me repeat that. After hitting the ball, the vast majority of its flight is determined by aerodynamics. This means that we must first understand how the air affects the ball, so that we can choose the best way to contact it for the few microseconds that we have.
What happens as the ball moves through the air?
When any object moves through the air, it moves the air too. Whole branches of physics are based on the movement of air and other fluids, but fortunately we don't have to dig too deep to understand enough for our context. The most important thing for golfers to understand is that air flows around the golf ball, following its curve.
Intuitively, this makes sense. As a ball moves through the air, it pushes air out of the way. On the front of the ball, a thin layer of air is trapped between the fast-moving ball and the slow-moving rest of the atmosphere. The atmosphere prevents the air from bouncing out in front of the ball, so it has no choice but to move quickly to the sides, along the surface of the ball itself.
Next, as the fast moving air reaches the top and bottom (and sides) of the ball, it again encounters slow-moving atmosphere, but now it is no longer pushed by the ball. Again, it is stopped by the atmosphere, and so "slides" along the outside of the ball.
Lastly (for smooth balls, at least) the thin layer of air finally separates from the ball as the ball passes it entirely. It still follows around the back of the ball slightly, because the ball "pulls away" from the atmosphere behind it, creating low pressure that draws the thin layer of air in, but not by much. As the air separates, it is still moving wildly and creates turbulence. Eddies (similar to whirlpools in water) form between the fast moving air that has just separated, and the slow moving air stuck to the back of the ball. These work together to slow the ball down.
The purpose of dimples
You may have noticed that golf balls are not smooth. Instead, they are covered in hundreds of shallow dimples. What on earth are these for? You may have heard that they cause the ball to travel farther, along with some explanation about reducing drag, and that is correct. In a bit more detail, the dimples cause turbulence in the thin layer of air surrounding the ball. This has the effect of allowing the air to stay in contact with the ball longer before separating from the back. This reduces the area of low pressure behind the ball, and reduces the size of any eddies formed. In turn, this means less drag pulling the ball backwards, and you get a longer flight.
What does spin do?
Every golfer knows what the effect of spin does to the ball. When the ball spins to the right, it curves right. When it spins to the left, it curves left. Ultimately, it means you missed (unless you wanted that slice). But how does spinning cause motion like that? Again, the answer has to do with the motion of air surrounding the ball, and it has a name: The Magnus Effect.
The Magnus Effect is the phenomenon that a spinning object tends to curve in the direction of its spin while traveling through a fluid (air, in our case). The explanation is relatively simple, but has a couple different parts.
First, suppose the ball is spinning to the right. This means that as it goes forward, the left side is moving forward faster than the right side is. The forward motion of the left side adds to the forward motion of the ball, but the backward motion of the right side takes away from it. Another foundation of fluid dynamics states that as the speed of a fluid increases, its pressure decreases. The result is that the pressure of the fast-moving layer of air is smaller on the left side and larger on the right.
Next think about how the air separates from the ball. Remember how the low pressure behind the ball drew the fast-moving layer in around the back a little? This new low pressure on the right side has the same effect. Instead of trailing a "stream" of air straight out the back, the ball now drags that air farther to the left side. Then, per Newton's Third Law of Motion, the act of dragging air left causes the ball to tend right.
Do Hooks or Slices go farther?
You may have heard that a hook (where the ball spins and curves to the left) travels farther than a slice, but is this true? To be honest, I'm not sure and I couldn't find a definitive answer. I would say probably not. A ball is symmetrical, spinning in opposite directions has the same effect in either direction, and so I don't see a difference that could cause the ball to travel farther or not. If there is a difference, it is probably due to how the ball gets spinning in the first place. To slice a ball, the club has to hit heel first, which means you are losing a little bit of leverage. Hooking the ball requires a toe-first hit, gaining a little bit of leverage. Either way, you probably missed.
Summing Up
Learning to hit the ball where you want it to go is the most important part of learning to golf. The stance, grip, and swing play major roles in hitting the ball, but these need to be adjusted so that you hit the ball the way you want in the first place. Knowing how you want to hit the ball starts with understanding how the ball behaves after you lose control of it. Fluid dynamics is a huge branch of physics, but there is no need to study more than the basics to get a handle on how the ball behaves in the air. Knowing how the golf ball's design and spin affect its flight path is essential to making decisions on how to hit it. Only after you know how you want to hit the ball can you focus on the hit itself.
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