Originally Posted by iacas
A week or two ago, a golf instructor posted this question to fellow instructors on Facebook.
Two golfers, one male, one female, have identical numbers on a launch monitor - launch angle, spin, clubhead speed, etc. Why does the male hit the ball 30 yards farther?
Several people wrote to say what's correct - that if their numbers (all
of their numbers for their impact conditions, including smash factor and ball speed) were identical, the balls would fly identical distances, and that's that. Something was wrong.
We later learned that the numbers were given to us from a range session, but the distances were from play on the course. Clearly the guy must swing harder or otherwise change his launch conditions on the golf course.
The thread was revealing, however. Science has taught us that the only thing that matters during impact is the clubhead and about 3-5 inches of the shaft. The collision between the ball and the clubhead are almost like two free bodies hitting each other. Nothing above the 3-5 inches up the shaft has any influence
Yet pros were chiming in to say "the male probably weighs more, and F=ma" or "the guy probably has a firmer grip" or some other things I've since forgotten.
Rubbish. We know this to be rubbish, or else Iron Byron wouldn't be able to keep a ball on the range. The simple (slightly over-simplified) reason: impact lasts so short an amount of time that the vibrations and deformations and whatnot that occur to the shaft don't have time to radiate up and back down the shaft very far. That's why only the bottom section of the shaft matters. It doesn't matter if you're squeezing the grip so hard you're cracking your shaft or if you literally let go of the grip at the precise moment of impact - the ball will go the same if the launch conditions are the same.
One of the more common - and frustrating - examples of "bad science" is F=ma. This formula - force = mass * acceleration - is used all the time to explain clubhead and ball interaction. But there are a few problems with this:
- EVERY clubhead decelerates when it hits the golf ball. It can't help but do so with any kind of golf shaft. Really simple physics there.
- If we consider the exact instant when impact starts, a clubhead traveling 100 MPH will hit a ball a certain distance. A clubhead accelerating will hit the ball within about an inch of a clubhead decelerating into impact.
Consider this: would you rather be hit by a car going 1 MPH but accelerating or the same car going 100 MPH but slamming on the brakes?
If you want to talk about clubhead/ball collision physics, you're better served (though not fully, of course, as neither the golf ball nor the clubhead are rigid bodies) using E=1/2mv^2. You'll note that only the mass (clubhead, bottom 3-5 inches of the shaft) and the velocity of the clubhead matters - there's no consideration for acceleration.
F=ma does apply to the golf swing, however, but the way it's often used is backwards. It would be more appropriate to see how a clubhead is accelerating and deduce the force required to make it do so. Of course, the problem with that is that F=ma is more of a linear concept, while most of the golf swing involves rotation (where an object that has the same linear speed is always "accelerating" because "acceleration" is a change in the velocity of an object - and velocity is both speed and direction
Another? Conservation of angular momentum. The classic example is that a skater goes into a spin. They pull their arms in tight and they spin faster, they put their arms out and they slow down.
COAM is used in the golf swing to describe why the hips slow down and the torso speeds up, then the torso slows down and the hands speed up, then the hands slow down and the clubhead speeds up.
The problem with using COAM is that it too is inaccurate. COAM applies quite nicely to closed systems (given the relatively low amount of friction on ice and due to the air, a skater is a reasonable example of this). The human body is not a closed system - we can use muscles to slow and speed things up.
And the most damning thing may be the very definition of the word "conservation." In science, "conservation" means "to keep the same." What is a golfer's angular momentum at the top of his backswing? Roughly 0. So if we conserved that angular momentum, the golfer would never make a downswing (or the parts swinging down would have to be offset by the parts swinging the other way - THAT's a swing I'd like to see!).
We can make a downswing because our muscles - which are "inside" the system (our bodies) - can continually add forces, change (increase) angular momentum, etc.
The point? There's a lot of junk science out there. Don't fall prey to it.