"Force Precedes Motion" and Flawed Physics in a TPI Video

[iacas]

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 it. The thing is… it's not true. In trying to simplify Newton's Laws of Motion, golf instructors frequently bungle it. There's value in simplifying things, but I reject simplification when it leads to a poor understanding.

In this video, Dr. Greg Rose and Dave Phillips (mostly the former) goof up on the physics of Newton's First and Third Laws of Motion. I'll explain why in this post.

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.

Rose, at the very least, left out not only the part about a body in motion staying in motion, but also left out a very important word: "net."

If you think back to your high school physics, when a bullet is fired horizontally from a gun, it begins falling immediately. There's no delay. The force (gravity) doesn't "precede" the motion (the bullet dropping) — it's acting on the bullet the whole time (even when it's sitting in the cartridge or traveling down the barrel of the gun) and the bullet begins dropping right away, as soon as the bottom of the barrel stops "holding it up." Rose saying that "force precedes motion" implies that things "wait" before moving like Wile E. Coyote floats before falling:

Rose does 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." Ehhhh, better, but still not quite right. It's an oversimplification that muddies the waters instead of clarifying them. Rose introduces two things that are oversimplified as well.

First, the idea of "movement." Physicists define it slightly 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. That's "movement" with no external force. But, physicists would say that  because your center of mass didn't move (physics definition), you didn't move anywhere. There's no "external force" acting on you there, but you're "moving" (common usage).

Second, 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 about a player's foot slipping, resulting in movement of the body despite a loss of ground reaction forces. The body moved in that scenario because of the muscles, or the internal forces.

Rose says "when you go to walk, you actually push into the ground." This is super nit-picky, but no… you don't. You push the ground horizontally, in a shear force direction. You're already pushing down into the ground because you're standing on it (gravity * your mass is doing it, really). 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), then move your foot and leg out to "catch" yourself before you fall on your face.

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.

Phillips then goes into a top of the backswing position and Rose correctly says that to move your right hip forward, your right foot actually tries to "pull" the ground behind you, away from the ball while your left foot tries to push the ground away from you, 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. Yes, that's 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 and you begin moving your center of mass upward. (Good golfers often generate 2x their body weight or more in vertical GRF.)

The shorthand “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 (within the right frame of reference).

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 stays 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, if you have friction with 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 movement (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 coded the lines to make it easier to see what's going on:

The top graph is their 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 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, and 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:

Also yes. No problem with these things — they're just 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 includes 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…" Then we get 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 things, then fumbled the ball at the 1-yard line on this specific part of the video. 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.

I use this example sometimes: imagine you have a frictionless surface, and a spring attached to an unmoving 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! 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!

---

I talk about this a lot with lateral forces. I prefer my golfers to shift to their trail side about 2-3" very early in the backswing, and then to shift forward toward their front foot around P3 (it 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 accelerate it in the other direction. The golfer pushes away from the target (green shaded area), then almost immediately begins pushing toward the target (magenta shaded area), to slow down and then begin moving forward.

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 every golfer begins pushing in the downswing direction before the downswing actually begins, and that's NOT an example of "force precedes motion."

There's no delay. 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” is a useful coaching cue, but it’s not literally true. In physics, force doesn’t sit around waiting for something to happen — if there’s a net force, acceleration (deceleration is negative acceleration, depending on the orientation of your reference frame) begins immediately. A more accurate way to say it is that force causes changes in motion, not that force somehow precedes motion.