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# Author : How-Hie (Tom) Ling

## Introduction

Before I talk about how to do the side spin serve and chop serve, I would like to introduce some physics involved in serving a ball and the aerodynamics of a ball in flight.  The reason I want to include these topics firstly is that by discussing the fundamental mechanics, I hope some serious minded players will develop the side spin and chop serves further or develop other forms of serves.  I developed and used the side spin and chop serves in late 1970s (among other moves and playing systems).  Since then, I haven’t seen any player doing real side spin serves.  A quick search on the internet and books showed that most publications covered under-arm serve, float serve and jump serve; and none on side spin serve and chop serve.

Disclaimer: The discourse below could be flawed as I am no expert in the rocket science stuff although I have studied fluid mechanics and aerodynamics some thirty years ago and co-designed a commercial catamaran in 1974.  I have been out of playing volleyball for twenty-five years, the serving actions described here are based on my memory.

## Force of the Serve (A Guesstimate Only)

Both the striking hand and the ball are soft objects, so the impact is semi-elastic.   The energy carried by the hand is transferred to the ball with some energy loss as heat
in deforming the hand and the ball.   The length of the impact is guesstimated by the author to be about 30 milliseconds (based on the impact of the tennis racket on the ball
of 5 milliseconds which I read somewhere).  After the hand hit the ball, the ball travels at
about 30m per second (for man).  The mass of the ball is about 10 ounces or 0.2841 kg.  Using the Newton’s Laws of Motions (v = u + at, u = 0), the applied force (f = ma) can be calculated as about 284 newton.  The maximum force is twice the average force, which is equal to 568 newton.

It is expected that the greater the force applied, the greater is the ball speed.  Also, the greater the speed of the striking hand (about 10m per second), the greater is the speed of the ball.  The mass of the hand (and lower arm) has some effect on the speed.  This is because the mass of the hand (about ¾ kg) and the forearm is a few times heavier than the ball (I think no body wants to lend a hand here to verify my assertion) J.  In Chinese, the striking force exerted is called jin(4), which loosely translates as fast-twitch-muscle plus strength.

(The maximum force exerted by a open hand calculated above as 568 newton is comparable to a black-belt karate chop whose force is estimated to be around 3000 newton.  Karate uses special technique to exert that force and is appropriate for breaking
boards and bricks only.  The force is not normally transmitted beyond the target.)

## Forces Acting on the Ball During Flight

For a perfect round ball moving through the air, it experiences three forces acting on it, namely, a drag force caused by air resistance which acts against the direction of motion, the gravity force acting downward and a lateral force called the Magnus force which acts at right angles to the direction of motion.

In the case of a not-so-round ball, it will also experience a net force due to unbalanced atmospheric pressure acting on its unequal surface areas.

## Drag Force and Reynolds’ Number

A ball moving through the air will have boundary layer (also called laminar) flow in front and a turbulent wake behind.  Since the wake is eddying, it possesses energy which can only come from the ball itself.  Thus the ball is continually losing its energy to this eddying wake, which causes it to slow down.  This eddying wake is the main cause of the drag force.  In normal circumstances, it is expected that the faster the ball is moving, the bigger is the eddying wake, and the greater is the air resistance drag.  (Actually, the air resistance increases as the square of the velocity).  However, at a certain critical value of speed, the ball will experience a sudden reduction in air drag to about one quarter of the previous value.  This is because at this high speed, the air in the front boundary layer becomes turbulent as well.  The result is that there is now some mixing of air in the boundary layer and the free air outside the stream.  There is now new energy coming from the outside into the front boundary layer which enables the air flow to flow around
the ball further back before the eddying wake occurs.  This means the wake is smaller and the drag is smaller as well.  (The dimpling of the golf ball is a good example of roughing the surface in order to produce turbulence further in front (thus less eddying wake at the back) so it can fly further).

In Fluid Mechanics, an indicator called the Reynolds Number measures the transition point at which the boundary layer flow turns into turbulent flow.  For a volleyball of about 21cm in diameter and travels at about 20m per second through the air, the Reynolds Number is about 268800 (RN = 640vd in normal indoor air), which is well beyond the standard critical number of 150000.  This means that a reduced air drag is experienced by most serves from speed of 12m per second to 40m per second or beyond.

The effects of variations in weather conditions have a minute effect on the air resistance.  The denser the air, the greater is the air resistance.  A rise in pressure will produce a proportional rise in density, while a rise in temperature will decrease the density.  A ball will fly slightly more quickly on a warm day and low pressure than on a cold day
and high pressure.

## Magnus Force and Magnus Effect

The above discussion assumes that the ball moves through the air without spinning.   A spinning ball will cause a different air flow pattern which will cause the ball to swerve from left to right, right to left, to float upwards or to dip down depending on the rotation of the ball.  We all know that a float serve (very popular nowadays L) tends to stay up longer in the air.  This is due to a mysterious but real force called the Magnus force acting upward on the ball.  In a float serve, the ball is rotating clockwise in a horizontal axis (as seen from a side view at position 5 corner), so the air flows faster at the top than the air at the bottom.  The wake is deflected downwards.  According to the Bernoulli’s Principle, the pressure at the top is now lower than the pressure at the bottom.  This means that there is a net force acting upwards which counters the gravity force.  This net force due to differential air flow pattern around a body is called the Magnus force.  This lateral movement effect is known as the Magnus effect.   The Magnus force is greatest when the axis of rotation is at right angles to the direction of the air flow.   It will be proportionally less when the angle between the rotation axis and the flow direction is less than 90 degrees.  When the axis of rotation and the direction of the air flow coincide, the force will be zero.  A rough ball will produce a more pronounced Magnus effect because the wake is dragged around further as explained in the previous Section.

(Heinrich Gustav Magnus was a German physicist who first explained the lateral movement of a spinning ball in 1852)

## Atmospheric Force on a Not-so-round Ball

Normal atmospheric pressure is about 10 newton per square cm in all directions.  The cross-sectional area of a volleyball is about 350 square cm.  Hence, for a perfectly round ball, it will experience about 3500 newton of equal and opposite forces on its opposite sides.  If, due to imperfect roundness of its sides, the forces were to be out of balance by 0.1%, then there will be a sideway force of 3.5 newton.  To compute how much a non-spinning ball will deviate from its original trajectory due to the unbalanced atmospheric forces acting sideway, we know that the mass of the ball is 2.841 newton.  This means there will be a sideway acceleration of 1.232 m per second squared.  In a typical flight lasting about 16/30 second (for a 30 m per second serve), using Newtons’s Laws of Motions (s = ut + 1/2a t2 , u = 0), the sideway movement at the end of the flight can be calculated as 1.75m.

Of course, one cannot hope to achieve a swing of 1.75m due to unbalanced atmospheric forces, but it can be deduced that with a minute imbalance in the sideway forces due to unequal surface areas, it is highly possible that the ball can deviate by 20cm at the end of the flight.   However, this deviation is gradual along its trajectory.

## Side Spin Serve

(The discussion below is for a person who serves right-handed.)
The side spin serve will make the ball swerve from left to right or from right to left due to the Magnus effect.  To swing the ball to the right – I called it the side-spin-right serve, you impact the ball with an open full hand at an area slightly to the left (2 cm) of the ball with a slight follow through to the right.  The effect will be more lethal if you are standing at the left corner behind the base line aiming at the opposite far right corner.  As the ball spins in a clockwise rotation (as seen from above) about a vertical axis of rotation, it will produce a Magnus force acting to the right.  To swing the ball to the left – I called it the side-spin-left serve, you strike with your open full hand at an area slightly to the right of the ball with a slight follow through to the left.  The effect is most marked if you stand at the right corner serving to the far left corner.

I normally served by bouncing the ball three times with my left foot forward and tossed the ball up slightly up and relied on my seeing (or proprioception) hand to strike the ball
while my eyes were looking at the net and the opponent court.  The wrist was tensed up (using jin4) so that from the finger tips to the forearm arm, the hand became one rigid weapon. The bend at the elbow ranged from a slight bend to 90 degrees.  As I hit the
ball, my right foot was brought forward to complete the serve.  A sideway movement of 30cm to to 60cm for the ball was easily achieved.

Due to the movement restriction imposed by the skeletal and muscular systems of a person’s right hand (for a right-hander who serves right-handed), the side-spin-right serve is easier to execute than the side-spin-left serve.  However, in outdoor court situations, with a cross wind blowing from right to left, in order to magnify the Magnus effect, you may have to serve the side-spin-left serves.  A softer ball will also produce more swing because you can compress and spin the ball better.  The compressed ball has the added effect of suddenly deflating to its original shape thus changing the path suddenly (normally during the last leg of its flight).  The differential atmospheric forces will also come into play on the compressed ball.

(A seeing or proprioception hand in volleyball is one which can aim at a target and hit the ball over the net to the target easily, much like a person can touch his/her index finger to his/her nose with his/her eyes closed)

## Chop Serve

The chop serve presents the edge of the hand to strike at the ball just like a karate chop.
Minute or no follow through with this kind of serve. The line of contact can be either center, to the left or to the right of the ball.  I normally tensed my wrist up and curved my fingers slightly in order to guide the ball in a certain direction.  The curved fingers have the effect of contacting the ball surface more than a straight edge hand.  It also protects the fingers from being hurt during contact if they happen to come into contact with the ball.

The impact of a chop serve will compress and distort the ball most because of the small contact area. This makes the ball unsteady in flight and has the effect of moving it side to side (about 10 cm) due to differential atmospheric forces acting.  I normally hit the ball off its center, so it swerved to one side as well as yawed in the air.

I also tensed my lump of flesh at the edge of my hand during chop serve (by tensing my wrist muscle).  This produced a hard edge which helped to hit the ball harder.

(The lump of flesh at the edge can be built up over years by striking the two hands together on their edges repeatedly.  This also helps in producing the jin4)

## Combination Serve

During 1970s, I had also tried these types of serves:
A side spin full open hand with a glancing chop serve,
A side spin hand with the folded-in thumb, and
A slanting chop serve.

Conclusion

Go and try it yourself.   Use your imagination.  Work at it.  Good luck to you all………..