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How do we create spin (or rotation)?
Spin is created by applying an external force outside the centre of mass
Where this eccentric force is applied will determine the way the projectile (usually a ball) spins
Where is spin used tactically in sport?
List four different types of spin
Give examples of reach type of spin
Heinrich Magnus followed Bernoulli in the early 19th century to apply fluid mechanics to rotating projectiles
The Magnus effect is the underlying theory of how an additional Magnus force can deviate a spinning projectile away from its expected flight path
Magnus effect: creation of an additional Magnus force on a spinning projectile which deviates from the flight path
Magnus force: a force created from a pressure gradient on opposing surfaces of a spinning body moving through the air
Can you think of any examples for when you have experienced Magnus force?
When used correctly the additional Magnus force can:
The Magnus effect works on the same theoretical basis as the Bernoulli principle
However, instead of a projectile's specific shape we consider the effect of the projectile's rotation or spin
The way the projectile spins determines the direction, velocity and pressure of air flow around it
Again, a pressure gradient is formed either side of the spinning projectile and an additional Magnus force is created which deviates the flight path
The pressure-gradient force is the force that results when there is a difference in pressure across a surface. A difference in pressure across a surface then implies a difference in force, which can result in an acceleration according to Newton's second law of motion, if there is no additional force to balance it. The resulting force is always directed from the region of higher-pressure to the region of lower-pressure
Velocity?
Pressure?
Forces created?
Air flow?
1. When the player kicks the ball off centre from the rotational axis of the ball, a anticlockwise spin is induced
2. As the ball spins through the air, the drag from air resistance on the right side of the ball increases while drag on the left side decreases due to the ball spinning around its rotational axis
3. This causes a drag imbalance causing the left-hand side of the ball to be a low pressure region as its velocity increase
4. This causes the high pressure region to move the ball to the left-hand side
The deviation created by the pressure gradient
means all forms of spin create a non-parabolic
flight path
What do you think happens to the flight path for each spin?
For a ball with topspin the additional Magnus force is
created by:
the oncoming air flow (top to bottom), which opposes motion,
decreasing the velocity of air flow - high pressure zone is created
The downward Magnus force adds to the weight of the projectile and the effect of gravity is increased
The projectile 'dips' in flight, giving less time in the air as the flight path shortens
In these sports, placing spin on the ball gives it stability in flight, guiding the air flow and reducing turbulence
The use of topspin shortens the flight path, meaning a player can hit the ball harder, thus ensuring it will still land in court or on the table; for example, a topspin serve in tennis or a topsin drive in table tennis
It can also confuse the opposition, bringing them closer to the net and unexpectedly putting them in a defensive position
If an eccentric force is applied beneath the centre of mass, backspin is created
Explain how the additional Magnus force is created on a backspinning ball and how this is used to a player's advantage in tennis and table tennis
Use the following prompts to guide you and sketch diagrams to add to your explanation:
Sidespin
Sidespin is also well used for making a ball swerve in flight, both hook to the left and slice to the right. These types of spin can be viewed easily from above, as shown below:
Free body diagrams and resultant force diagrams can also be drawn to show the forces acting on a spinning ball in flight and the resultant force using the parallelogram law.
Biomechanists then use these diagrams to analyse performance of both athletes and equipment
Tiny changes in the surface of a
ball, for example, can have a big
impact in their flight through the
air - for example, for a ball with
backspin
When drawing an air flow diagram for a ball with topspin or backspin it is viewed from the side whereas sidespin is drawn from above
Airflow diagrams must show:
not the whole clip, just bits with practice shown
The effects of topspin and backspin in tennis extend to the bounce of the ball:
How can the equipment used impact the rotation of the ball in play?
Research one of the below and describe how you think impacts the rotation?
Jabulani football
Dimples in a golf ball
Softball instead of baseball
Spin is most well appreciated on the football pitch
Research the Adidas Jabulani football from the South Africa World Cup in 2010 and compare it with the performance of the redesigned Brazuca ball used in South America in 2014
Consider their performance, stability and reliability as a spinning projectile and their effect on the game for both players and spectators