## October 22, 2012

### Swing of Cricket ball : The Magnus Effect

BALL SURFACE
If during the play one side of the ball roughs up and other side remains smooth. Then swing occurs
SWING
Swing is swerving of cricket ball towards left or right as it moves through air.  This happens when ball spins about its vertical axis or because it spins about an axis perpendicular to the seam.
The fast bowlers swing the ball by making the seam inclined at angle of 15° to 25° to the direction that the ball is headed such that the smooth portion of ball is above & in front.

In this way the air flows smoothly around the smooth half but it becomes turbulent on other side as it has to flow past the seam.
The turbulent air is at higher velocity. Therefore according to Bernoulli’s theorem the pressure on these side (rough) decreases than on smooth flowing air therefore the ball gets pushed sideways.
This is also the Magnus effect. This sideway swing force is maximum when speed of ball is around 100 km/hr. But it speed of ball increases.
The reverse swing occurs because the flow of air over smooth surface becomes turbulent.  The effect can occur at low speeds if rough end side is put forward
Other factors: affecting swing are the atmospheric pressure: - Due to evaporation of ground moisture.  The water vapor content of air over pitch increases. This increases the partial pressure and thereby air pressure therefore as match progress the fast bowlers will be able to get more swing if they manage roughness/shine factor well.

OUTSWING
Holding the ball for an out swinger with shiny/wet side on right side

The ball initially moves out in the air and comes back to the batsman sharply.  Usually, a batsman can be out by leg before as the tends to play along wrong line.

Holding the ball for an in swinger, with shiny side on the left side

The ball initially moves towards the batsman and at last moment darts away from the batsman. A batsman can be caught behind the wickets and sometimes bowled.
STREAMLINE AND TURBULENT FLOW
of a liquid is that flow in which every particle of the liquid follows exactly the path of its preceding particle and has the same velocity in magnitude and direction as that of its preceding particle while crossing through that point.
When a liquid moves with a velocity greater than its critical velocity, the motion   the particles of liquid becomes disorderly or irregular. Such  a flow is called turbulent flow.

Curved motion of spinning ball (Magnus effect)
When a spinning ball is thrown, it deviates from its usual path in flight. This is due to magnus effect..
If the ball is spinning as well as moving linearly, the streamlines at the top of ball due to two types of motion are opposed to each other and those below are in the same direction. As a result of it, the velocity of air flow is greater below than above the ball.  Now, according to Bernoulli’s principle, the pressure on the upper side of the ball becomes more than the pressure on the lower side of ball. Due to it, a resultant force F acts upon the ball at right angle to the linear motion in the downward direction, resulting the ball to move along a curved path. This dynamic force due to spinning of ball is called Magnus effect.

Hope this post is helpful ....