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"5 Minutes Daily" Practice Challenge
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Day 26 (16 Jul 26) - worked with putting mirror focusing on start line - putting from 8-10’ on the practice green. -
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Day 315 7-16 Arm wide in downswing. Working on a little more trail forearm rotation as well. -
If you're not into physics (or nitpicking at physics) as it applies to golf, you can just stop reading right here. If you are, keep reading! I have a few problems with the video above. The title of the video gets to the topic: the idea that "force precedes motion." It's a statement that, if you hang around golf instructional conversations long enough, you're probably going to hear. The problem is… it's not true. In trying to simplify Newton's Laws of Motion and apply them to golf, instructors frequently bungle it. While I understand that there's value in simplifying complex things, I reject simplification when it leads to a poor understanding or untrue statements. In this video, Dr. Greg Rose (who does most of the talking) and Dave Phillips goof up on the physics of Newton's First and Third Laws of Motion. I'll explain how. Right away, Rose starts with the "notion" that "force precedes motion," which he then calls "Newton's First Law." That's not true — Newton's First Law of Motion is: A body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a net force. If you think back to your high school physics, when a bullet is fired horizontally from a gun, it begins accelerating downward (falling, dropping) immediately. There's no delay. The force (gravity) doesn't "precede" the motion (the bullet dropping) — it's accelerating the bullet downward the whole time, even when it's sitting in the cartridge or traveling down the barrel of the gun. The bullet accelerates downward due to a net force as soon as the bottom of the barrel stops pushing upward. Rose saying that "force precedes motion" implies that things "wait" before moving like Wile E. Coyote floats before falling: Rose does almost immediately restate Newton's First Law as "objects at rest will remain at rest unless there's some type of external force that makes them move." Rose not only left the part about a body in motion staying in motion, but also left out a very important word from the First Law: "net." This is a better definition of Newton's First Law, but it's still not quite right. It's an oversimplification that muddies the waters instead of clarifying them. Rose introduces two flaws: First, the idea of "movement." Physicists define movement differently than the common usage. Imagine that you're floating in outer space and the only forces really acting on you is a negligible amount of gravity (from the sun, Earth, Jupiter, a far-away black hole… etc.). You can "move" (the common usage) a finger, an arm or a leg, or bend forward at your waist without any external forces acting on you. But, your center of mass would not move (accelerate your arm one direction, and the rest of your body will accelerate a bit slower in the other direction or something). Physicists would say that because your center of mass didn't move (physics definition), that you didn't actually "move" anywhere. Second (and of less specific relevance to the general topic here), the idea of internal and external forces. Rose says that "we can't move unless some external force makes us move." Again, I can "move" by using my muscles. They are what "cause" the movement. I gave the outer space example above, and Rose himself will later talk about a player's foot slipping during the golf swing, resulting in movement of the body despite a loss of ground reaction forces. The body moved in that scenario because of the muscles, not because of external forces. Rose says "when you go to walk, you actually push into the ground." I'm going to be super nit-picky here, but no… you don't. To begin walking, you push the ground horizontally — a shear force. You're already pushing down into the ground because you're standing on it (gravity * your mass is doing it, really), so you don't begin walking by pushing down into the ground. Have you ever heard the idea that walking is repeatedly falling and catching yourself? To begin walking, you actually lean forward a little bit (applying a small shear force in the opposite direction), gravity begins pulling your center of mass downward in an arc around a pivot point in one of your feet, and you move your other foot and leg out to "catch" yourself before you finish accelerating toward the earth (falling). 🙂 Rose says "one of the principles that we always like to talk about is that the force happens before you start to move." No! It does not. This is not true. Phillips then poses at a top-of-backswing position and Rose correctly says that to move your right hip forward, his right foot actually tries to "pull" the ground behind him, away from the golf ball while his left foot tries to push the ground away from him, toward the ball. That is correct, and we call that A/P force (anterior/posterior). Phillips says "to do that, you've gotta push in the right direction," at which time (1:45) Rose says that "now you're bringing up Newton's Third Law," which he then says is "there's an equal and opposite reaction." No! Newton's Third Law of Motion is: If two bodies exert forces on each other, these forces have the same magnitude but opposite directions. Rose gives the example that if you push down with 100 pounds, the ground pushes back with 100 pounds. That is true… but that's not particularly relevant. If you weigh 100 pounds, but you push down with 200 pounds, the ground also pushes back with 200 pounds of force, but you are overcoming the force of gravity (100 pounds) and so you begin accelerating your center of mass upward. Immediately. (Good golfers often generate 2x their body weight or more in vertical GRF.) The shorthand version “For every action there is an equal and opposite reaction” is common, but it often leads people to think that one force causes another. That’s not what Newton’s Third Law says. The two forces are part of the same interaction and exist simultaneously. They are equal in magnitude, opposite in direction, and act on different objects (each other). For a golf-related example, when a golfer pushes against the ground with their lead foot during the downswing, the foot exerts a force on the ground. At the exact same time, the ground exerts an equal-magnitude, opposite-direction force back on the golfer (the ground reaction force). These two forces are a Newton’s Third Law pair. Notice that they act on different objects: one force acts on the ground, the other acts on the golfer. For another, it doesn't matter to the physics at all if you swing a driver at a stationary ball… or propel a ball at a stationary driver: the physics and the reactions will be the same. For a non-golf related example… if you stand on a dock and push a boat away with your hands (or your foot), you exert a force on the boat. Simultaneously, the boat exerts an equal and opposite force on you. The result is that the boat moves away from the dock while you are pushed backward. Again, the forces are equal and opposite, but they act on different objects. Again, Rose properly says that to move your right hip forward and your left hip backward, you must try to push the ground in the opposite direction. Since the ground won't move (its mass is a little bit bigger than your own, and Chuck Norris has sadly passed away), you move as a result of the interaction (which is also why, if you have enough friction between your shoes and the dock, why the boat moves and you aren't pushed back much). This (around 2:18) is also when Rose mentions the golfers slipping… in which case there's not an equal and opposite reaction, because we have a net force causing acceleration (and movement via slippage) through loss of friction — the ground is no longer pushing back horizontally enough to stop your foot from moving. Rose then shows a graph (there's a reflection on it so I grabbed the best screenshot I could): I've colored the lines to make it easier to see what's going on: The top graph is the golfer's lead (left) leg, the middle graph is their trail (right) leg, and the bottom graph is the "pelvis rotation." Greg doesn't say what kind of "pelvis rotation" graph it is, but from looking at it, I think we can assume it's the angular (rotational) velocity of the pelvis, as if it was the actual angle of turn, the golfer would reach the end of the follow-through with a pelvis right back where it started at address. That seems unlikely. 😄 Rose states correctly that when the lead leg "goes negative" the left foot is pushing forward and the GRF is pushing backward (away from the ball), that the trail leg goes positive, away from the ball, and the GRF pushes the right hip forward, toward the ball. Rose has his assistant move the playback forward to this point: I've added a vertical yellow line through the graph at that point to show it: Rose says "the first thing [this golfer does] is push with the right leg backward so the ground starts to push [the right hip forward]." Yes. Phillips then says "it happens this early" and points at the skeleton avatar here, just past P2 in the backswing: Also yes. I have no problem with these statements or the graphs/measurements. The assistant advances the swing a few more frames, and Rose says "now all of a sudden comes the left foot." Rose then says at about 3:40, "because everything's rotating [in the backswing direction], they need to start to create these forces to stop the rotation." Yes! Then at 3:50, Rose adds "the forces have already happened, but notice this is pelvic rotation" (he points at the bottom graph). "Pelvis is still rotating negative. When this (bottom graph) goes positive, your pelvis is rotating forward." Phillips says "which is huge, because most people do not understand this." Given this video, "most people" may include Rose and Phillips! 😛 At 4:07, Rose again says "they're starting to create this A/P push in the backswing to slow down the rotation…" YES! But then he continues with "Let's go all the way to when the pelvis starts to rotate forward…" The pair shares this exchange: Rose: "I want you to notice how much earlier did the forces start?" Phillips: "Way earlier." Rose: "Way earlier. Forces precede motion." No! Like Leon Lett, Rose was saying some good and correct things, then fumbled the ball at the 1-yard line with "forces precede motion." The "motion" that the forces created where the yellow line exists is, as he said twice, to SLOW the rotation of the pelvis in the backswing direction! (Pedantic note: the forces accelerate the hips in the downswing direction. Forces cause accelerations — positive or negative depending on how you've oriented your reference frame.) Here's a simple example: you're coasting in a car down a gentle hill. You apply the brakes. The car doesn't immediately stop, of course: the brakes do immediately apply friction in the opposite direction, accelerating (or negatively accelerating if you want) the rotation of the wheels. The forward motion down the hill continues for a bit, but the negative acceleration (braking) is applied immediately upon the brakes being applied to the brake cylinders. I use this example sometimes with golfers who understand a little physics: imagine you have a frictionless horizontal surface with a spring attached to an unmoving vertical wall. You slide a block along the surface and it contacts the spring. The spring begins pushing against the block immediately, but the block doesn't change direction right away. It compresses the spring a bit, the forces are unbalanced, and the block slows down (it could be negative or positive acceleration depending on which direction you've set up as positive). When the block reaches a speed of zero (for an instant), it begins accelerating in the other direction as the forces remain unbalanced, right up until the block leaves the spring and slides at a constant speed (the speed at which it hit the spring if the spring is "lossless" as we often assume them to be in simplified physics test questions) because the forces are again balance (no net forces anywhere). Rose says "what's about to happen is a result of the forces that happened before." No! It's already happened. If those forces in the downswing direction didn't already happen, the golfer's pelvis would have kept turning in the backswing direction! Rose: "What did the great player do? They started turning earlier. They started creating the resistance earlier because they're going to use those forces to come out of the backswing with speed, they're not going to start the downswing with force." Once again… No! No! No! I talk about this somewhat often with golfers regarding their lateral forces. I prefer that most of my golfers to shift to their trail side a few inches very early in the backswing, then shift forward toward their front foot around P3 (this varies depending on the golfer, the length of the backswing, etc.). I'm going to show you the lateral movement graph from one of the first golfers I had on my Smart2Move 3D Dual Force Plates. In the graph below, the red line is the contribution from the right foot, the blue line is the left foot, and the yellow line is the sum of the two. Negative is the golfer pushing away from the target, positive is toward the target. I've stopped the graph at the first moment where the graph reads as net positive — the golfer pushing toward the target: What direction is the golfer moving here? Away from the target! It's really, really early in the backswing that the golfer begins pushing toward the target: Why? Because if he didn't, he'd continue to sway away from the target. The spring begins pushing back against the block immediately, first to slow it down, then to move it in the other direction. The golfer pushes away from the target (green shaded area), and accelerates away from the target as long as the yellow line is negative, then almost immediately begins pushing toward the target (magenta shaded area), to slow down the movement away before they begin moving forward (when the red area under the curve surpasses the green area under the curve). Just like the golfer in the TPI video above, and just like EVERY GOLFER ever. The difference between great players and poorer players? The timing of when these things happen, the magnitude of the forces, and the relative balance of those two things for parts that involve both feet. But I guarantee you that every golfer begins pushing in the downswing direction before the downswing actually begins They have to, or they'd keep going in the backswing direction! This is NOT an example of "force precedes motion." There's no delay — when we apply a net force, we cause acceleration instantly. This results in a change to the motion — the object in motion doesn't continue at the same speed in a straight line anymore. A common misconception in golf instruction is to identify the force of a golfer against the ground as waiting on the "reaction force," or as viewing it as an “action followed by a reaction.” In reality, neither comes first or second — they occur at the same time. They are the same interaction viewed from opposite perspectives, occurring at the same instant. The phrase “force precedes motion” can sometimes be a useful coaching cue, but it’s not actually true. In physics, force doesn’t sit around waiting — if there’s a net force, acceleration (or negative acceleration, depending on the orientation of your reference frame) is immediate. A more accurate way to say it is that net forces causes accelerations, which can change motion.
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By gestate0560 · Posted
Listening to episode 71 on golf equipment and I have to admit I never fully got the importance of grinds and bounce and sole shape generally. Why would it matter if you’re hitting ball first?
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