Spin on the Ball: What Causes Spin At Impact (Lesson 5, Part 1)

Lee Westwood spins a ball into a green.

Lesson 1 discusses the importance of consistency in golf; Lesson 2 the importance of lining up a putt accurately; Lesson 3 getting the ball to the hole; and Lesson 4 golf as a game of opposites.  We will go back and forth between these lessons.  Now we are ready to introduce the first part of Lesson 5, which concerns spinning the ball during the full swing.  This is a very difficult topic, in general, but it is central to understanding how to strike the ball, how to strike it well, and how to strike it consistently.

About this site: Golf As God Gave It To Us

Felix Frankfurter was one of President Roosevelt’s New Deal Justices.  When the old vanguard that sat on the Court when Roosevelt first entered began retiring, Roosevelt appointed justices like William O. Douglas, Chairman of the Securities and Exchange Commission, and Felix Frankfurter, a noted Harvard Law School professor.  Frankfurter is famous in the law for many things, including his concurrence in Cooper v. Aaron and his dissent in Baker v. Carr.  A friend once told me that Frankfurter used to say, “I don’t much like analogies; first I have to figure out why they are different; then I have to figure out why they are the same.”  That is why I like analogies.  You have to think.

To learn about putting spin on the golf ball, we are going to start with a game with a much bigger ball – soccer.

Why a soccer ball?

You don’t hit a soccer ball with a stick or a club or a bat.  But in a manner of speaking you do.  Your leg in soccer works very much like the club in golf.  And the advantage of taking a soccer ball is that it is very much like a golf ball in that it is round, symmetrical, and you try to strike it from a place – ideally – that you control.  It is also very different from a golf ball:  it is much larger; it is struck by an object that is much larger; and it is easier, and harder, to compress. (We will leave compression out of the equation at this point.  It would make our discussion of spin much more difficult.)

Moreover, given that a soccer ball’s equator is always fairly high off the ground, it is much like a teed up golf ball. We can eliminate in this dicussion, at least in this post, the complexities created in golf by hitting a ball that is sitting on the ground — terra firma.

Beckham makes a corner kick, from where he can score goals!

Bend it like Beckham

The saying in soccer clubs is “bend it like Beckham.” If you are American, you have probably watched a few soccer games.  And you’ve seen a corner kick, where a player from a team gets to try to kick the ball into the opposing teams goal area.  The best kicks curve into the area.  The bend of the ball is not accidental.  One might think it is.  It is not, anymore so than a hook or slice is accidental in golf – except when we mis-hit a ball we intended to strike differently..

An idealized soccer ball, marked for east-west and north-south hemispheres.

The idea is to make it difficult for the defenders to predict how the ball will enter the area in front of their goal, and this depends upon it having left-to-right or right-to-left spin. What is amazing, at least to me, is that one can actually score a goal with a corner kick — a kick that is made from a point along a line that is even with the goal.  In the notes below, [1] and [2], we have given links to examples where a soccer ball is kicked and hooks or slices.  In one of the examples, a professional player scores a goal with a corner kick.

The two main factors.

Two principal factors in a soccer kick (and golf swing) influence the type of spin put on the ball.  One is the direction the club is travelling relative to the place on the ball that is struck. The other is the center-of-gravity of “that” which strikes that ball, relative to the ball and where it is struck, and when discussing “that” we have to be a bit careful.  To simplify at first, let’s imagine that the ball is being struck by a “point mass” traveling in a certain direction.  This will help us understand spin better.  Then we can add in the second factor in a later post

— that is, we will put aside for now the relative centers of gravity (or center of mass) of that which is doing the striking (a foot) and that which is being struck (the soccer ball), and we will focus on the direction in which the ball is struck relative to its surface.

Hook spin.  In the next diagram, we have taken our idealized soccer ball and spun it a bit, so to speak, in our diagram.

The quadrant on a soccer ball we would like to imagine ourselves striking, if hitting it as a right-footed kicker trying to impart hook spin.

We want to emphasize, for a right-footed striker, hitting the ball in the place where we are most likely to create right to left spin, all other things being equal.  For this spin to be created, the direction of our kick must be “across” the ball.  That is, kicking it in that quadrant, as highlighted, supposes that we intend to kick from that quadrant across the hemisphere line of the ball.  If we apply force straight down the hemispheric line, the ball should go straight.

It is easy to see that  that if our foot is moving across the ball, and is below its equator, that we will not just move it forward in the direction of the strike, but part of the velocity of our kick will cause the ball to rotate right-to-left around its axis.  Let’s take an overhead view to make this as clear as possible.

….when the ball is struck at an angle that is not perpindicular to the ball’s surface at that point, the ball gains an angular velocity, 𝛳.

Drawing the velocity vectors, we can see that a kick that is slightly across the surface of the ball (an angle to it rather than perpindicular to it at that point) will cause the ball to rotate.  Diagrams 1 and 2.  It will have a forward (linear) velocity, v, and and angular velocity, 𝛳, as in diagram 2.

Diagram 1:  A soccer ball struck at an angle not perpindicular to the ball’s surface

Why does the soccer ball actually spin through the air, rather than move on a straight line, while spining about its axis? (Extras for Experts)

Good question.  The careful reader may wonder why the spin on the ball results in spin in the air.  For this, we have to ask our consultant, D.R.B. “Backspin” Newly (sometimes known as D. Bernoulli).  This is one of the more interesting aspects of sports, to my mind.  It is also why airplanes fly, and curve balls curve.

When the ball is rotating in one direction or the other, it pushes the air on the opposite side by faster, while the near side of the spin runs into the air, so to speak.  One side of the ball therefore literally pushes the air by faster, while the other side slows it down.

Near the surface of the ball, this creates a pressure differential, applying Bernoulli’s law.  [1] (Bernoulli didn’t really write the law; he just discovered it.) You may have heard of the phrase, laminar flow, which you can read more about here in a fairly straightforward fashion, complete with examples from football – American football. As we shall see at some point, laminar flow around a dimpled golf ball is what gives it its basic flight characteristics – that is, lift, drag, distance, leaving aside side-spin, the subject we are now discussing.

Diagram 2: ….when the ball is struck at an angle that is not perpindicular to the ball’s surface at that point, the ball gains an angular velocity, 𝛳., and a linear velocity, v.

Faster moving air has a lower pressure; slower moving air exerts greater pressure.  In this way the air will push the ball from the high pressure side of the ball to the low pressure side.  If the ball is spinning left to right, it spins to the right.  If the ball is spinning right to left, it spins right.

To fully understand this point, it is worth point out that, if we were to kick the ball in outer space and put side spin on it, the ball would not curve.  It would just move forward with a linear velocity imparted by the foot, and a angular velocity imparted by the degree to which the foot is contacting the ball with a sideways motion in relation to the surface of the ball at the point of impact.

There are no curve balls, slices, or hooks, in space. For the very curious, additional detail and examples from every day life are provided in this link.

Our angle of “attack” on the soccer ball and golf ball.

The flight of a soccer ball struck at (1) an angle to the surface of the ball imparting right to left spin, and (2) an angle to the intended target line to the left of the target line, on the ball, results in (3) a ball moving forward while spinning about its axis from right to left, which (4) travels through the air with a “hook” or “draw” spin to it.

What we now have seen is that if we hit the soccer ball exactly perpindicular to its surface, at the place we strike it, it will go perfectly straight.  If we strike the ball at an angle to the surface, it will start to rotate in the direction of that angle, as well as move forward too.

If a soccer ball is struck in the direction represented in diagram 3 by the velocity vector, v2, which comes into the ball slightly to the left its intended path, moving out toward the right, the ball will move forward andhave be imparted with left-to-right or “hook” spin.  In fact, it should, much like a draw in golf, start out to the right of its intended path (because the force is being implied across that path) and spin back to the left across that path (just as we’d see with a draw on the golf course).  In sum:  The ball should hook (in golf terminology).  On the other hand, if the soccer ball is struck along the line of its north-south hemipshere line, and moving in that direction, as in Diagram 4, we would expect to see the ball move “straight” and with little to no sideways spin.  Again, we are assuming here that the “thing” striking the ball is a “point mass.”  That is, that it only touches the ball at the point of impact and no where else.  That is not the case in golf or soccer.  The idea is, however, to eliminate a variable (the shape of that thing that strikes the ball) so we can isolate, and see easily, cause and effect of different ways of coming into a soccer ball relative to its intended line of flight.

Diagram 4: A ball struck at an angle and at a direction that likely will make it fly “straight”

This is interesting for golfers.  For one, we see that we can have an inside-out swing, for example, and if the clubhead is “open” to the line of intended flight, the ball we simply go straight — to the right.  Why?  Because, as shown in Diagram 1 (arrow perpindicular to the ball surface), it will be (from the ball’s perspective) as if the club is square to an “inside-out” or “left-to-right” line for a right-handed golfer.  The more we are coming inside-out and the ball is struck at an angle to its surface that is “closed” as we think about it, the ball will get hook spin and will move through the air from right to left.  (Diagram 1, arrow an angle to the ball surface.)

Once more — and this is also important to know — you can be moving inside-out on your swing, and if the clubface is open enough, it will strike the ball at an angle that can create right to left spin — a push slice.  That’s an interesting thought.  Why is that?  Because an “inside-out” or “outside-in” club path are not “open” or “closed” relative to the intended line of flight, but to the angle at which the club path is going to strike the ball — that is, at the point of impact, will the path of the club be to the left, or to the right, of a line at a 90 degree angle to the surface of the ball at the point of impact.  This is what is key.  Not “inside-out” or “outside-in” relative to the intended target line.   You can experience this most easily, if you swing inside-out by putting the ball a couple inches back in your stance with a wood of some kind.  It will very likely go high and right.

A golf ball with hook spin.

(Most golfers who come in to out tend not to have the clubface open much at impact – certainly relative to the path of the club; one most often sees a fairly straight ball in these situations which appears to have some “slice spin” and drops off to the right at the end of its path.)

Slice or fade spin …. why all this makes consistent striking of the ball important… backspin, top-spin, spin and dimples on the ball … and how you can use of this understanding in your day-to-day play … all good topics for the future.

Conclusion

We have, for now, said quite a bit about spinning the ball.

Experts in Spin — But Not Spin on A Ball

So we will stop for now and let folks absorb, and return to the subject soon, when we write another part of this lesson on the subject of putting spin on the ball during the full swing.  Remember, right now, we are only focusing on the point where ball and clubhead meet – not how the clubhead reached its destination with the ball.  This, what happens at impact and spin of the ball, is by itself a complex enough topic.

A. Einstein

A. Einstein

© 2012

[1]

Megan Rapinoe scores a corner kick goal during the Summer Olympics in a match against Canada. [Video from NBC.]

[2]

David Beckham scores a goal on a direct kick by “bending” the ball into the goal in a 2012 MLS match. [Youtube Video.]

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This entry was posted in (Linear) Momentum, Angular Momentum, Ball Flight, Bernoulli's Principle, Fades and Draws, Geometry of Golf, Gravity and Golf, Hooks and Slices, Opposites in Golf, Other Sports and Golf, The Mechanics of Golf. Bookmark the permalink.