table tennis presentation

Magic behind table tennis

Table Tennis racket

Blade

MATERIALS!• 85% by thickness must be natural wood

• Other layers made of materials such as CARBON FIBRE, ARALYTE, or glass fibre.

• CARBON FIBRE - to strengthen and stiffen the blade, to increasing the speed and the size of the sweet spot of the blade.

• ARALYTE - also to increase the size of the sweet spot, to dampen vibration and to give a softer feel than carbon.

Rubber

• Adhesive layer• 7.5% or 0.35mm

pimples-out rubber surface

Aggressive players• thinner •To shorten the impact time between the ball and the racket so as to increase the force. The ball will then able to move in faster speed, increasing the chance for the opponent to miss the ball.

Defensive playersThickerTo lengthen the impact time between the ball and the Racket so as to decrease the impact force so it is easier for The player to return the ball in case of facing an aggressive player

The materials used affects the followings:

• Weight• Balance• Speed

Weight

Head size• Air resistance between large or smaller headed blades –

insignificant

• Larger rackets require more rubber more rubber to cover the surface heavier

• Lighter blades- swung more quickly• Heavier blades- more mass to hit the ball.

In physics terms…

momentum = mass velocity

Balance

• The centre of gravity of the blade is closer to the handle or the tip of the head

• Aggressive players- head-heavy blades - extra spin and speed

• Defensive players- blades with the centre of gravity towards the handle- increase the feeling of control

CENTRE OF GRAVITY DEPENDS ON THE WEIGHT

Speed

The speed of backspin ball will decrease after hitting the table.

The speed of the top spin ball will increase when hitting the table

Why is the 38mm ball faster than the 40mm ball?

• Though the mass of the 38mm ball is smaller than 40mm, it is faster.

• When considering the very small mass, air resistance become a main determining force.

• The 40mm ball will experience a greater wind resistance and hence and lower speed.

How to increase the forceon serving the ball?

In view of energy changes in table tennis

Throw the ball into air

• With initial energy input

• Compared with starting with a static ball

• Have kinetic energy before served by the player

• Result in larger velocity, acceleration and force

Initial KE + PE loss= KE of the ball before hitting back

The Law of Conservation of Energy

Energy in a system may take on various forms (e.g. kinetic, potential, heat, light). The law of conservation of energy states that energy may neither be created nor destroyed. Therefore the sum of all the energies in the system is a constant.

Energy loss in table tennis

• Friction against the ball when the ball hits the table

• Not perfectly elastic collision

• Air resistance acting on the ball throughout the trajectory

• Effective on this light weight

• Result in asymmetrical trajectory

Torque

• Torque – rotational moment that occurs when a force is applied at an angle around a fixed point

The first step for spinning

by the friction between the ball and the racket

Inside the table tennis ball

• Rotate about a point inside it

• The larger the torque, the faster the spin

How to increase the torque applied?

• Unwind the body to increase the radius for the torque

• Hit the ball at the outer rim of the racket

• Hit the ball harder, hence increasing the velocity, then acceleration and so the force

About the trajectory of table tennis ball in the air

Projectile Motion

When considering simple strike, the ball move in simple projectile motion.

• Without spinning, the ball moves in a straight line when you observe from the top.

(Even it performs projectile motion) • Bernoulli Principle states that:

Lowering of velocity results in increasing of pressure.

• If the ball spins, the trajectory is bent.

• The main reason is that air is viscous and the surface of the ball is rough.

• When the ball spins, it drags the air around it.

• The air flow on the side A has a greater velocity than that at B.

• According to the Bernoulli Principle, the pressure at A is small than that at B.

• A net force F is acted on the ball.

• The trajectory is then bent.

• If the spin is in vertical.

• Velocity at A is higher than that at B.

• The pressure at A is small than that at B.

• A net force F is acting downwards on the ball.

• The ball comes down rather quickly.

• The locus is no longer a parabola.