How High Will It Fly? : Background Information

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The Flight of a Ball

Sports balls come in many sizes and colours with a variety of surface coverings. Different balls have fabric, stitches, dimples, or textured covers. Most sports balls have evolved over the history of their sport. Their appearance and performance are due in part to the equipment and surface which the ball hits (a racquet, a bat, grass, a wood floor) and the distance the ball must travel.

The goal of the particular ball game or shot may not be to travel the furthest or the fastest. You will lose the point in volleyball if you serve the ball so hard that it lands outside the court. In some sports points are won by reaching a specific location or target, or by making your opponent miss points. In sports like cricket and baseball, the ball may be pitched using clever techniques to fool the opponent into thinking the ball will travel in a certain direction or land in a different spot.

The athlete's sheer power is not the only factor which determines the direction and distance a ball travels. You may be surprised to learn that a ball's surface seams, cover, size, spin and the altitude at which you play all affect a ball's flight.

Forces and Motions

In the game of "tug-of-war" or in arm wrestling, you pull or push, trying to overpower your opponent. Who will win? Whoever can apply the most force. What if you played "tug-of-war" with three small people pulling against two large people? Who would win? It really does not matter how many people are on a team: the group that pulls with the most force will win.

When you are pulled or pushed in many directions at the same time which way will you move? For example, imagine that you are walking your dog on a lead into a strong wind. Your dog decides to chase a squirrel and pulls you to one side. Which way will you go: forward, backward or to the side? To find out you need to consider all of the forces acting on your body. If your dog is a large St Bernard, you will probably be pulled to one side. If your dog is a tiny Chihuahua, you will probably be pushed backward by the wind.

How does this apply to the flight of a ball? Once a ball is struck, hit or thrown by a player the laws of science guide its flight path. Different types of forces act on a ball at the same time. These forces are: the weight of the ball, the lift, and air resistance (also known as drag). The path the ball will follow (the trajectory) and its speed depends on the strength and direction of each of these forces.

Speed and Velocity

Speed is a measurement of how fast an object is moving. Speed is calculated by dividing the distance travelled (a length) by the time it takes the object to move that distance. The dimensions of speed are length per unit of time: for example, metres per second (m/s) or kilometre per hour (km/hr). If a person runs 5 kilometres in half an hour, his or her speed is 10 kilometres per hour (km/hr). If a tennis ball travels a distance of 20 metres in 0.5 seconds its speed is 40 metres per second. The "velocity" of an object is a measure of both its speed and direction.

Forces on a Sports Ball

Weight

The weight of a ball is a measure of how heavy or light it is. Weight is a force and is dependent on the object's mass (a measure of the amount of matter an object contains) and gravity.

Generally, a sports ball's weight and its size are regulated by the sport's governing body. Usually, the weight and size can vary only within a small range. For example, a standard tennis ball may weigh between 56.0 grams and 59.4 grams; its diameter may vary between 6.541 cm and 6.858 cm.

Lift

Lift is created by the movement of the air around an object. On an aeroplane or a bird, most of the lift is created as air moves over the wing's unique shape. On a sports ball lift is created by spinning the ball. The word "lift" is a little misleading, because one meaning of the word is "to rise". Normally, on an aeroplane or bird that means moving "up". In the science called "aerodynamics" (the study of how liquids and gases move), lift is a force which has a very specific definition and does not always mean "up". For example, race car designers create wing-like surfaces on the car to generate "negative lift": a downward directed force. This force and the weight of the race car help the driver maintain control on the high-speed curves of a race track. Lift is dependent on the velocity (speed and direction), density and diameter of the ball.

A ball can spin in many directions. Relative to the ball, in topspin the top of the ball spins forward (top to bottom) into the oncoming air. For backspin the bottom of the ball spins backward (bottom to top). Balls can also have sidespin. Some sports have special terms for different types of spin. In volleyball a serve hit with no spin is called "flat", while in baseball a pitch with no spin is called a "knuckleball". In tennis backspin is called "underspin". In golf and tennis a ball hit with sidespin is referred to as a "slice".

Topspin

The famous scientist, Sir Isaac Newton, discovered that for every action there is an equal, but opposite reaction. (This is called Newton's Third Law of Motion.) When you blow up a balloon and let go, the air rushes out of the back of the balloon as the balloon shoots forward. This is an example of an opposite but equal reaction. To understand lift on a ball let's apply Newton's Third Law of Motion.

Here are photographs taken in a wind tunnel. A wind tunnel is a device that engineers use to calculate the forces a gas, like air, creates on an object. In this particular test, smoke was blown over a tennis ball and you are able to see the pattern the air made as it moved over the ball.

In the first photograph the ball is not spinning. Look at the wake on the right side of the photograph. It is right behind the ball.

In the next photograph the ball is spinning with topspin. Look at the wake on the right side of this photo. You can see that this air is moving upwards at an angle. The air is applying an upward force on the back of the ball. Remembering Newton's Third Law this means that the ball will be forced down. Topspin pulls the ball down faster - its lift is in the negative direction.

In the last photograph the ball is spinning with backspin. The wake on the right side of the photo is directed down. This means the ball will be forced up.

What about flat (no spin) balls? There's no spin, the wake is not directed up or down - so there's practically no lift!

For the athlete, this means that a ball struck or thrown with a certain amount of force with backspin will go further than a flat or topspin ball. The ball with backspin is lifted up in the air higher than a ball hit with the same amount of force using topspin or with no spin. Likewise, a ball with no spin will travel further than a ball hit with topspin. Tennis and volleyball players make use of this fact when they serve. To make sure that the ball does not go up too far or outside the court lines, they will serve with topspin since it will not go as far as a ball with backspin or no spin.

Red ball has backspin; green ball has no spin; yellow ball has topspin.

The ball with backspin travels further than the other balls.

Drag

Drag is a retarding (slowing) force which acts on any object moving through a fluid (a liquid or a gas). Boats, submarines and swimmers moving through water as well as aeroplanes, birds and sports balls flying through air all experience drag, which slows them down.

The shape of an object and its surface roughness affect a sports ball's drag. Smooth balls (like a cricket ball) have less drag than a ball with a rough surface like a tennis ball. The fabric covering on a tennis ball is very rough and a great source of drag. Many competitive swimmers shave off their body hair to reduce drag. Cyclists wear body-fitted rather than loose baggy clothes to reduce drag. In other sports such as automobile racing, large financial investments are made to streamline the shape of race cars to reduce drag.

In addition to surface roughness and shape, drag, like lift, is dependent on the velocity, density and diameter of the ball.

Altitude and Air Density

Air is not as dense (the air molecules are not as close together) at higher altitudes. If you have ever visited a higher altitude you may have become tired when you exerted yourself. You became tired because there was less oxygen in the air you breathed at high altitude. Aeroplanes carry their own air for passengers and the cabin you sit in is pressurised for the same reason.

The flight of the sports ball is also affected by altitude. Lift and drag are dependent on the density of the air. When the density of the air is lower, the lift and drag are lower and a ball, generally, will travel further. This is a major concern for athletes who travel all over the world and play in cities at different altitudes. It is also very important in court sports. An athlete who hits a good ball just "in" the court at sea level, will find that their ball will be "out" on a court at high altitude.

SportSim

As you use the SportSim sports ball simulator you will experiment with different sports balls, the altitude, velocity and spin rate. You will be able to see the different flight paths created by different sports balls as you change these parameters.

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