Armswing question

[RobbieB]

Mostly for coaches/proshop gurus to think about...
If a bowler has a free swing- ie, no muscle tension/acceleration in the swing - what effect do the following changes have on ball speed and why?
1- Raising or lowering the position of the ball in the stance;
2- Increasing/decreasing ball weight.

Cheers, Robbie.

 

[weggy]

I'm going to say that a higher pushaway will result in a faster ball speed. This is because the higher potential energy from the pushaway will result in a longer backswing - thereby increasing the potential energy for the followthrough. In contrast, starting with the ball almost hanging vertical will yield a smaller potential energy.

The second point is interesting though. On first thought, the heavier ball would create more downward momentum and provide more potential energy. However, as the ball comes past the midpoint of the swing, it would be likely to slow down sooner than a lighter ball. So in this case, I'll tip that there is no difference when changing ball weight in a free swing.

So as summary:

1. Higher ball position = faster ball swing due to gravitational potential energy
2. No difference because the heavier ball slows down sooner than the lighter one but gains more initial speed and therefore the difference cancels.

 

[jason_doust]

Mostly for coaches/proshop gurus to think about...
If a bowler has a free swing- ie, no muscle tension/acceleration in the swing - what effect do the following changes have on ball speed and why?
1- Raising or lowering the position of the ball in the stance;
2- Increasing/decreasing ball weight.
Cheers, Robbie.

Ooo... What a good geeky question! It's like when two trains leave at the same time and so on... I suspect that you're seeing just how far down this rabbit hole we go, Robbie ;-)

1-Raising or lowering the ball should raise or lower the swing height all things being equal. However, all things aren't equal. The "pendulum" hangs off a moving fulcrum (the shoulder), who's motion is in turn affected by the motion of the pendulum. This is not simple maths, but we won’t need to do the calculus to work out the result.

As the ball swings back, it drags the shoulder back (open), reducing the forward motion of the shoulder and its attached body. As the shoulder only closes in the final moments of the swing as the ball drops to the release point, when the body's forward motion is decreasing toward zero in the slide and the horizontal vector of the swing becomes small enough to overcome for mere mortals, it’s safe to assume that this loss of velocity is not recovered.

Therefore, higher “swing drag” of the body is created by the higher ball position, owing to higher momentum needing to be reversed at the top point of the swing. A ball moving faster backwards takes longer to slow, leading to a higher swing and later timing unless foot cadence is slowed to accommodate the longer swing time.

However, the ball is now set to drop from a higher point. This means that the greater arc of the downswing spends more time in the zone where the vertical component of the swing vector is greater than the horizontal component, meaning that gravity is assisting the acceleration of the ball. This is serious assistance at 9.8 m/s². (Gravity becomes more neutral as the swing bottoms out and becomes more forward than downward.)

Therefore, in an "unmuscled" swing (don’t know, never had one to speak of), the higher ball position leads to a higher backswing peak at the point of the power step than a lower ball position would achieve at the same point in time, so we can safely assume that the long held belief of a higher ball position leading to greater delivery velocity holds true.

In fact if you want to prove it, go to your nearest park and play on the swings. Feel the difference in your velocity swinging from different heights.

2- Increasing/decreasing ball weight. Now this one is trickier.
Increased weight means increased momentum (mass x velocity); therefore more of the aforementioned drag on the body as the ball moves from a backward to forward motion. The energy for this change in direction has to come from somewhere and chances are it comes from your shoulder and torso which are propelled by your legs. Ipso Facto - your body gets slowed down somewhat more by a heavier ball.

What the momentum view of things doesn't take into account is gravity - or why we all get saggy as the years go by. Galileo is said to have dropped spheres of different weights from the top of the leaning tower of Pisa to demonstrate to skeptics that the spheres would strike the ground together. (http://www.newscientist.com/article/mg14920146.300.html) Assuming that the laws of gravity haven't changed in the last 400 years or so, a heavier ball should in theory fall at the same rate as a lighter ball and likewise rise at the same rate. If you had a truly pendulous swing, no difference to ball speed would occur.

In bowling though, a number of variables come into play.

• Players will either pitch the lighter ball, making it go faster;
• the lighter ball will not push as hard into the lane surface, making it travel faster down the lane;
• or the player will lift or rotate the lighter ball a bit more, as it offers less resistance to the wrist and fingers, making it travel slower down the lane, owing to greater hook.

Difficult to tell what the heavier/lighter ball will do without knowing the individual’s game to be affected by the weight change.

If you're not confused yet. Congratulations. My nerd propeller hat has just about burnt out a bearing on this one...

Cheers,
Jason

 

[weggy]

Yes, I am somewhat confused now Jason! But congratulations on putting so much detail into your answer

I am going to post an addendum to my answer which may make it easier for non-scientific bowlers to understand.

The formula for a simple pendulum (which is what we have with a free swing) is:

T = 2π√l/g

Where T is the period (time to complete a full cycle), l is the length of the pendulum (or arm in this case) and g is the value for gravity, which is a constant value. 2π is also a constant value in the equation.

The only part of the delivery that makes any difference, according to this formula, is the length of the bowler's arm (which is also constant), therefore the time taken for a complete cycle (backswing and followthrough) remains the same in both of these cases.

1. Because the time taken isn't changed, the ball must move faster when using a higher pushaway because it has further to travel in the given time than a lower pushaway, proving what has already been said.

2. Again, the weight of the object has no bearing in the movement of a simple pendulum. In this case, assuming the pushaway is consistent with both weights, the ball speed should (will) remain the same.

An interesting question Robbie, something for people who are looking at either of these measures to think about.

 

[jason_doust]

Hi Weggy,

Now, what if you're Pete Weber or a very good clone of him and you have a controlled upswing, which becomes remarkably free on the downswing?

Cheers,
Jason

 

[RobbieB]

OK guys,
First one - Weggy is exactly correct. A higher backswing = more energy = more speed, all else being equal. The second question is a bit more complicated. A heavier ball will swing SLOWER than a light ball.

Why? - because the arm has a weight of its own. The pendulum formula that Weggy quoted is one that everyone who does high school physics will have seen, but it only applies to a pendulum with a light (ie weightless) string. Because your arm has a weight, the centre of gravity of the pendulum is not the centre of the ball, and the lighter the ball the more the centre of gravity moves up the arm. And a shorter pendulum swings faster.

Cheers, Robbie.

 

[jason_doust]

Couldn't agree more, except that have you ever met anyone with a perfect pendulm swing? They'd have about zero finger leverage. Once you introduce any kind of throwing action to the shot, as most folks do, things start getting a lot more complex.

 

[weggy]

Hey Robbie

Good to see you took notice in high school physics! The formula only works for weightless 'strings' as you say. However we are talking about the difference when a bowler changes these variables.

I would have thought that since the bowlers arm is (hopefully) the same weight in all cases, this formula would still work. In this case, it would not matter that the arm has weight.

And Jason... that is a good point about **** Weber. Bowling's tougher than it sounds, eh guys?

 

[jason_doust]

Hi Folks,

What Robbie's referring to isn't high school physics. More like 2nd year Engineering. The mass of the arm does matter, as it affects the location of the centroid (CG of an irregular object), which in turn must affect the period (time for one swing) of the pendulum. Shorter distance, faster period, faster swing. Just what those guys with muscles need... The laws of Physics on their side too! This finally makes sense out of some big fellas I've bowled with over the years who had dinky backswings and good, even great ball speed. They made it look so easy and it used to drive me nuts! Thanks Robbie!

Applying a little "push" to the top of the downswing, like swinging your girlfriend on a swing at the park creates "resonance" you just add something to gravity. Even David Ozio (who really has one of the best swings ever) reckons that it's about 65-70% pendulum, with the remainder being muscle application through the swing and release.

But Robbie, once again, thanks for a good question. I've just learned something new!

 

[RobbieB]

For a pendulum with a massive 'string', e.g. an arm, the length is the distance from pivot to the centre of mass of the pendulum, including the arm. Increasing ball weight moves the cg down the arm, making the 'string' longer.

BTW, for a real world pendulum the period is dependent on the angle it swings through, at odds with the classic formula. This is because the formula is based on an approximation that only works for small displacement angles. So starting the ball higher lengthens the period of the swing for a free swing.

 

[weggy]

Ok, gotcha. Some people have way too much time on their hands! But again, a good question to think about.