Transcript spin. - School of Physical Education

Spin Impact and
Elasticity
By
Dr. Ajay Kumar
Reader
School of Physical Education
DAVV Indore
Definition & Types of Spin
Rotation around any axis is known
as spin.
Rotation around horizontal axis is
known as topspin or back spin.
Rotation around vertical axis is
known as side (left or right) spin.
Cause of Spin
The cause of spin is the application
of off center force.
The off center force is the force
which does not pass through the
center of gravity of the implement.
Due to spin the path of the ball also
changes because the ball is moving
in air, so aerodynamics effect is
also a factor which is to be studied.
Properties and Nature of Spin
 A ball moving through the air will also
move in the direction of least air
pressure.
 If the ball is spinning it will follow a
curved path.
 Ball with top spin drop sooner than the
ball with no spin.
 A ball with back spin will move in an
upward curve and thus stay aloft longer
than a ball with no spin.
Properties and Nature of Spin
(Cont)
 Balls spinning around a vertical axis
have side spin.
 Right spin causes the ball to curve to the
right and occurs when the forward edge
of the ball moves to the right.
 Left spin is the opposite of right spin.
 The amount of the air a ball drag with it
when spinning is depends upon the
surface of the ball and speed of the spin.
Properties and Nature of Spin
(Cont)
 Rough or large surface, small mass and a
fast spin speed will all produce a more
noticeble spin and curve deflection.
 The small mass of a table tennis ball, fuzz
on a tennis ball and the seams on a
baseball all enhance spin.
 The deflection will also be more
pronounced if the forward velocity is
slow.
Properties and Nature of Spin
(Cont)
 Spin on a ball may also smooth its flight
by acting as a stabilizer.
 The first and foremost use of the spin is
spin provide accuracy.
 Spin also change the rebounding
properties of the ball.
Example
 Spin in Table tennis
 Spin in Tennis
 Spin in Cricket
 Spin in Volleyball
 Spin in Baseball
 Spin in Discus
 Spin in Javelin
 Spin in Football
 Spin in Basketball
Impact and Elasticity
The nature of impact is governed by
– Elasticity
– Mass and velocity of the rebounding
surface
– The friction between the surfaces
– The angle with which one object
contacts the seconds
Momentum and Collisions
Momentum helps us understand
collisions/ impact
Elastic Collisions - objects rebound
Inelastic Collisions - object stick
together and usually become
distorted and generate heat
Elasticity
 Any time two or more objects come into
contacts with each other some distortion
or deformation occurs.
 Whether or not the distortion is
permanent depends upon the elasticity of
the interacting objects.
 “ ELASTICITY IS THE ABILITY OF AN
OBJECT TO RESIST DISTORTING
INFLUENCE AND RETURN TO ITS
ORIGINAL SIZE AND SHAPE WHEN THE
DISTORTING FORCE ARE REMOVED.”
Elasticity (Cont)
 The force which acts on an object to
distort is called ‘stress’.
 The distortion which occurs is called
‘strain’ and it is proportional to the force
causing it.
 Stress may take the form of tension, as in
the stretching of a spring, compression,
as in the squeezing of a tennis ball,
flexion, such as the bending of a fencing
foil, or torsion, as in the twisting of a
spring.
Elasticity (Cont)
In all the previous case the objects
tend to resume its original size and
shape when the stress is removed.
If the stress is too large the elastic
limit of the object a permanent
distortion occurs.
Elasticity (Cont)
The material such as rubber seems
highly elastic since it yeilds easily
to a distorting force and returns to
its original shape.
Actually, substances which are hard
to distort and return perfectly to
their original shape are more elastic.
Gases, liquids highly tempered
steels are examples.
Elasticity (Cont)
In comparing the elasticity of
different substance, coefficient of
elasticity are used.
A coefficient of elasticity or
restitution is defined as the stress
divided by strain.
The coefficient of elasticity most
commonly determined in sports for
a ball is
Elasticity (Cont)
e=
√
_____________
Bounce
___________
height
Drop height
e = coefficient of restitution or elasticity
Coefficient of Restitution
v 2x  v1x
e
u 2x  u1x
For perfectly elastic collisions e=1.
If the two object stick together, e=0.
Otherwise 0<e<1.