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 For perfectly elastic collisions e=1. If the two object stick together, e=0. Otherwise 0<e<1.