Sasho Mackenzie - Newtons 2nd Law F=Ma (misconceptions in golf swing)

[Wildthing]

For anyone who remembers their old school physics lessons where their teacher used to teach by rote (which my one did and didn't really understand physics at all) this might be confusing.  But one can clearly see how easy it is to misinterpret Newton's 2nd Law when applying it to the golf swing. One can end up with a completely wrong perception on how to hit the ball a long distance.

https://twitter.com/i/status/1333958039049744387

Dr MacKenzie polled some questions regarding Newtons 2nd Law  on his twitter below and there are some surprising results.

 

Edited December 3, 2020 by Wildthing

[Wildthing]

Is there anyone out there who has a background in physics and maths?  I'd like to exchange some private messages with them concerning some issues I don't understand about the golf swing kinetics. 

Example : I have looked at  Dr Sasho  MacKenzie's  'Intro To Club Kinetics' VIMEO video below (see up to 06:00 ) and the physics looks wrong. 

https://vimeo.com/158419250

Sasho says  'Eccentric Linear Net Force'  will cause a 'Moment of Force' (ie. Torque) that will rotate the clubs  COM (centre of mass)  to line up with the net force  (see image below).

I have plotted the movement of the COM and its a straight line , not rotating at all .

Also in physics , an eccentric force (one that doesn't pass through the COM)  on a rigid object causes translational movement and a 'couple' .  As far as I am aware, a couple will cause rotation of a rigid object around its COM (Centre of Mass) . A 'couple' is a free vector and effectively causes the rigid body to spin around its COM and not cause it to accelerate.

The only forces that can rotate the COM of the clubhead is a 'Torque' and that has to be applied via the hands but its NOT that 'Moment of Force'.  Other movements by the golfer are somehow being directed through the arms and hands to cause the clubs COM to rotate in the downswing. 

Unless I'm mistaken, the speed of the clubhead is caused by :

1. Linear forces (mainly via increased tension in the shaft)  that will accelerate the COM  and clubhead in a straight line . This can be increased by a sudden variation in hand path, with the COM moving in one direction while the hands move in another.

2.  'Intrinsic Spin' of the club around its COM (ie. the 'Couple' caused by the 'Eccentric Net Linear Force' ). This will cause the clubhead to spin around the clubs COM increasing its speed.

3. Torques via the hands (not just active muscular wrist torques although that is a possibility) that move the COM (and therefore clubhead)  around in a curvilinear path. 

All three above are contributing to clubhead speed but its mainly point 1 responsible for clubhead speed in a golfers downswing. 

Have I made any obvious errors?

 

 

 

Edited December 5, 2020 by Wildthing

[cksurfdude]

I _do_ like physics and math - or at least I used to many many many years ago - but I'm thinking your best conversation partner for this stuff is Bryson!

[Wildthing]

When I look at   'Moment Of Force'  graph below (black line) it is  therefore representing  the 'spin' of the golf club around its COM , not the clubhead (or its COM)  rotation around the 'Mid-Hand Point'.

Further , 'Hand Couple' (blue graph)  is also a 'free vector' and will also cause the club to 'spin' around its COM.

What we really need superimposed on the below graphs are 3  more graphs . 

1. Speed of clubhead sweetspot vs time

2. Speed of COM of club vs time

3. Net linear force applied by the hand vs time 

Then we might be able to relate what proportion of 'couple' and 'force' might be contributing to the 'clubhead sweetspot speed'  during the downswing .

 

Edited December 7, 2020 by Wildthing

[Wildthing]

The graphs above are really describing the movement of the club in space as per diagram below, from top of the backswing P4 to impact P7.  They are describing the 'spin' of the club around its COM.

So basically the 'Hand Couple' and the 'Moment of Force' (which is also a couple ) has spun the club from P4 to P7  in the time taken to perform the downswing. 

Using my own driver characteristics :

The COM is about 11 inches from the base of the club (ie. length AB =R) = 0.28 Metres  

Point B will have rotated 270 degrees from P4-P7 = 4.7 radians

For a typical pga pro, the  downswing  time is about 0.25 secs

So the average angular velocity (caused by the couples) = 4.7/0.25 = 18.8 rad/sec

using V = RW   (ie. speed = radius x angular velocity)

Average clubhead speed (caused by the couples only) in the downswing= 0.28 x 18.8 = 5.3 metres/sec = 11.9 mph

Lets make another  large assumption that most of the 'spin' happens around 100 milliseconds from impact (maybe P5.5), so lets use this as the downswing time rather than 0.25 secs

The average angular velocity (caused by the couples) =  4.7/0.10 = 47 rad/sec

Average clubhead speed in the downswing from P5.5-P7  = 0.28 x 47 = 13.16 metres/sec = 29.4 mph

So it's rather amazing that  the golfer is actually 'spinning' the clubhead  to such a high speed by impact using 'Couples' . 

The average speed of a PGA golfers driver is 114.1  mph so where is the other 114.1 -29.4 = 84.7 mph coming from ?

It has to be coming from linear forces applied via the hands on the clubshaft  pulling the clubhead (using increased shaft tension) from P4-P7.

 

 

 

 

 

 

 

 

 

Edited December 7, 2020 by Wildthing
apologies -made lots of spelling and grammatical errors