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………..
