INTERESTING VIDEO ON THE PHYSICS OF RUNNING

Physics of cricket

                  Swing bowling

A cricket ball can swerve to the left or the right as it moves through the air, either because it spins about a vertical axis or because it spins about an axis perpendicular to the seam. Vertical axis spin is commonly used by spin bowlers by not by fast bowlers. Fast bowlers prefer to swing the ball by making sure the seam is inclined at an angle of about 20 degrees to the direction that the ball is headed, in such a way that about 3/4 of the front of the ball is smooth. That way, the air flows smoothly around the smooth half but it becomes turbulent on the other side since it has to flow past the seam. Turbulent air is at a lower pressure than smooth flowing air, so the ball gets pushed sideways. It is almost impossible to eliminate backspin as the ball leaves the bowler's hand, but if the spin axis is perpendicular to the seam then it will help to keep the seam aligned at a fixed angle.

The sideways force on the ball peaks at about 110 km/hr, drops to zero at about 120 km/hr and then reverses direction. Reverse swing arises because the air flow on the smooth side becomes turbulent at sufficiently high ball speeds. The smooth side then becomes the low pressure side so the ball swings in the opposite direction. Normally, this effect is significant only at speeds above about 140 km/hr. However, the effect can occur at lower speeds if the ball has a roughened side and if the roughened side faces forward. Conventional swing bowlers polish the ball so one side is as smooth as possible. Reverse swingers like to make sure the other side is as rough as possible. The best ball to swing is therefore one that stays smooth on one side and roughens up during normal play on the other side.

Details of the aerodynamics involved are described on my home page under the heading Ball Trajectories where you can find several pdf files to download on the subject, including one called Sports Balls.pdf and one called Fluidflow Photos.pdf. The secret behind swing bowling lies in the way that the thin boundary layer of air near the ball surface can separate from the ball either early or late depending on whether the air flows smoothly over the surface or is tripped into turbulence by the seam or by roughness of the surface, or both. Those boundary layers were revealed many years ago by the marvelous smoke tunnel photos shown in the Photos.pdf file. Here is one taken by the late Professor F. Brown from University of Notre Dame showing how air flows around a sphere when part of the bottom half is covered in a rough grit. Air separates early over the smooth portion, becomes turbulent over the rough portion and separates later, so the air is deflected upward, resulting in an equal and opposite downward force on the ball. That is the secret that lies behind almost all aerodynamic flows, and it is what determines both the lift and drag coefficients acting on an object. Note how air backflows into the low pressure  “hole” left behind the ball, forming a turbulent wake

Physics of curling

CURLING

Physics Of Curling — Temperature And Humidity

To ensure optimal curling conditions, the surface temperature of the ice is maintained at around -5 degrees Celsius. In addition, the humidity of the air in the rink is deliberately kept low in order to prevent condensation of water on the ice, which can adversely affect play conditions.

The temperature of the running band of the curling stone is kept at the same temperature as the ice to ensure proper play. If the running band is warmer than the ice surface, increased molecular bonding will result. This will increase friction with the ice, which will adversely affect play.

This concludes the discussion on the physics of curling.

Physics of Basketball

Basketball

Physics Of Basketball — Backspin

Backspin is used by players to improve their chances of getting the basketball into the net. When an object is spinning and bounces off something, it will have a tendency to bounce in the direction of the spin. This is useful for players who bounce the ball off the backboard, or the back of the net. The resulting bounce will more likely send the ball downwards into the net. Without backspin the ball is more likely to bounce away from the net.

Physics of Ice Hockey

Ice hockey

The physics of hockey is a broad subject of analysis, covering key aspects related to performance and equipment design. Experienced hockey players are generally aware of (either directly or indirectly) how physics plays a role in their ability to play the game. The purpose of this page is to discuss how the game of hockey and physics go together.

Note that the information presented below is largely based on the book by Alain Haché: The Physics of Hockey, and the website: http://www.thephysicsofhockey.com

The book by Alain Haché goes into greater detail on the physics of hockey than I do on this page. If you want to purchase this book, click on the image link below. You will be taken to the Amazon website, where you can make your purchase.

Physics Of Hockey — Skating

For the most part, hockey physics, as related to skating is the combination of the skates gliding over the ice while also pushing off the ice with the edge, in order to gain speed. Experienced hockey players make this combination of movements look like second nature, which is to be expected since they must be able to play offensive and defensive positions, while also controlling the puck.

The physical properties of ice is what allows hockey players to maneuver the way they do. For instance, the low friction of the skate blade with the ice is what allows a hockey player to easily glide over its surface. And the physical make up of the ice is what allows a player to dig in with his skate in order to go around a turn, speed up, or stop.

A hockey player propels himself forward by pushing off the ice with a force perpendicular to the skate blade. Since the friction of the blade with the ice is almost zero, this is the only way he can propel himself forward. The figure below illustrates the physics behind this principle.

As the hockey player pushes off with his rear leg, a perpendicular force Fis exerted on the skate by the ice. The component of the force F that points forward (in the direction of motion) is what pushes the player forward. At the same time, his other skate is either raised or gliding on the ice. As he moves forward he then switches to the other leg and pushes off the ice with that one, and the process is mirrored. To push off the ice with greater forward force (and accelerate faster), the skater increases the angle α, which increases the component of force in the direction of motion.

To avoid turning their backs on the opposing team, hockey players sometimes skate backwards using a gliding pattern in the shape of a lazy "S" (as shown below). In this skating pattern, the player's blades never leave the ice. However, the player cannot push off against the ice as hard as he does when skating forward, which means he cannot go as fast.

In this technique, the player pushes against the ice with his push-skate facing inward, while his other skate glides. As he moves backwards he then switches to the other leg and pushes off the ice with that one, and the process is mirrored. Thus, the physics of hockey related to skating backwards is similar to that of skating forward.

As a hockey player gains speed, the velocity of his pushing leg relative to the ice decreases. This reduces the amount of push force that he can exert on the ice. For instance, if a hockey player can at most move his feet at 7 m/s then the greatest push force will be when he begins skating from rest. At this point the velocity of his foot relative to the ice is 7 m/s (a maximum). As the player gains speed this relative velocity decreases. For example, if he reaches a speed of 5 m/s, the relative velocity of his foot relative to the ice is 2 m/s, and the push force is less as a result. Consequently, there is a maximum speed a hockey player can reach, which is directly influenced by how fast he can move his feet on the ice. However, the maximum speed the player can reach is not necessarily 7 m/s. To determine the maximum possible speed we must look at the biomechanics of the player on the ice. The biomechanics of a player as he moves on the ice is another useful analysis in the physics of hockey, related to skating.

To maintain his balance when accelerating forward, a hockey player will crouch forward in the direction of motion. This prevents him from falling (tipping) backwards due to the torque caused by the forward component of the force F. By crouching (or bending) forward, the player is moving his center of mass forward which creates a counter-torque. This counter-torque balances the torque caused by the forward component of F, and this prevents him from falling (tipping) backwards.

The design of the hockey skate is another important factor related to the physics of hockey, on the equipment side. A hockey player's blades must be able to support his quick acceleration, turns, and stops. This is accomplished by grinding a slight hollow into the bottom of the blade. This creates two sharp edges which "bite" into the ice, and prevent slipping. The figure below illustrates this.

Due to heavy use during a typical game, a player's blades must be regularly kept sharpened in order to maintain optimal performance. If the edges become dull the result can be a player's foot slipping out from under him as he goes around a turn or attempts to stop.

Physics Of Hockey — The Puck

A hockey puck is made of a hard vulcanized rubber material, able to withstand the high level of wear and tear during a game. They are colored black in order to be highly visible against the surface of the ice.

Hockey pucks are frozen prior to being used in a game. This reduces the level of friction the puck has with the ice and allows it to travel further on the ice, without "sticking". This is convenient from a player's point of view since he prefers to maintain his momentum on the ice without having to stop and hit the puck again to get it moving.

Freezing the puck is also done to intentionally reduce how much it bounces during play. This enables better control of puck movement.

Physics Of Hockey — The Hockey Stick

The hockey stick has several different features illustrating the physics of hockey (as related to the equipment side). The hockey stick is designed to enable good puck control, while also being lightweight and strong enough to withstand the stresses placed on it during use.

One of the key features of a hockey stick that affects puck control is the curvature of the blade, which acts as a type of self-centering mechanism. When the puck is struck the curvature of the blade "forces" it towards the bottom of the curve, where it tends to sit for the brief duration of impact before flying off. This allows a player to make more consistent shots since the puck tends to fly off the same part of the blade every time.

However, for the sake of fairness and uniformity of play, regulations typically limit the amount of curvature a player's blade can have. For example, in the NHL the maximum curvature is defined as follows: The perpendicular distance from a line drawn from the heel of the blade to the end of the blade, and to the point on the blade of maximum distance, shall not exceed three-quarters of an inch. A diagram of this is shown below.

In the above diagram the maximum allowable distance denoted by the blue line is three-quarters of an inch.

Some players curve their blades closer to the end of the blade, which makes it a bit easier to scoop the puck away from another player. Personal preference is a main factor in how players curve their sticks.

Curvature of the blade also allows players to more easily put spin on the puck which gives it gyroscopic stability during flight. This makes it more likely that it will land flat on the ice. Applying tape to the blade improves friction between puck and blade. This aids the ability to put spin on the puck.

Another feature of a hockey stick that affects puck control is the "loft" or "face" of the blade. This is the tilt angle of the blade, visible when looking at the stick from directly above. This is illustrated in the figure below.

A greater tilt angle makes it easier for a player to lift up the puck and get it airborne. Again, how much tilt works best comes down to the personal preference of the hockey player.

A feature built into hockey sticks, tailored to a player's style of play, is the angle of "lie". This is the angle the blade makes with the shaft. This is represented by the angle θ in the figure below.

Players usually seek a lie angle that will put their blade flat on the ice while they are in their typical skating stance.

As shown in the figure above, the toe is the very end of the blade. The toe comes in two basic shapes: round and square. The difference between the two is that the round toe allows more ability to control the puck at the tip, while the square toe increases the blocking area at the tip.

A good choice of material for hockey sticks is carbon fiber. It is lightweight and has high strength. This is important for ease of puck control and for making shots, such as the slapshot (which will be discussed next). This is a good example of how material science is an important part of the physics of hockey.

Physics Of Hockey — The Slapshot

In the slapshot, players can clock puck speeds of over 100 miles per hour, making it the hardest shot in hockey.

The hockey player begins the slapshot by raising the stick behind his body, as shown below.

Next, the player violently strikes the ice slightly behind the puck, and uses his weight to bend the stick, storing energy in it like a spring. When the face of the blade strikes the puck the player rotates his wrists and shifts his weight in order to release this stored energy and transfer it to the puck. The result is the puck reaching a speed faster than it would if the player simply hit the puck directly. The kinetic energy of the puck after impact is equal to the stored energy in the hockey stick.

The figure below shows the point of impact between the stick and puck. You can clearly see the bend in the stick.

Thus, the physics of hockey taking place here is the transfer of energy from player to stick, and from stick to puck. The advantage of storing energy in the stick is that (upon release) it strikes the puck faster than the player can, causing the puck to reach a greater speed.

Physics Of Hockey — The Goaltender

The role of the goaltender is to block shots made by the opposing team. To protect his body from injury he wears protective gear to absorb the impact of the puck with his body. Due to his high level of exposure to high-speed pucks he wears even more protective gear than the other players. However, the weight and bulk of the gear can slow down and restrict the movement of the goaltender somewhat. Thus, strength and cardiovascular training, as well as the learning of good technique and efficiency of movement, is an essential part of being a good goaltender.

Physics Of Hockey — The Lacrosse Style Goal

This is a particularly interesting subject in the physics of hockey. In the lacrosse style goal, the hockey player skillfully maneuvers the puck into the net, while maintaining contact between puck and blade. The picture below shows Mike Legg who, in 1996 (while playing for the University of Michigan), scores a lacrosse style goal from behind the net. The approximate trajectory of the puck from ice to net is represented by the blue curve.

To begin the move, Mike Legg orients the puck on its edge so that it touches the blade of his stick head on. He then guides the puck along (using the blade of his stick) such that it follows a curved trajectory, as shown. This curved trajectory causes the puck to experience centripetal acceleration. The centripetal acceleration points towards the center of curvature of the curve, in the direction of the red arrows (shown in the picture). This centripetal acceleration in turn causes the puck to "push" against the blade hard enough so that it doesn't fall off due to gravity.

There are basically two things that must happen in order for this trick to work:

(1) He must move the puck along the trajectory at a high enough speed (v) to generate a high enough centripetal acceleration (a_c), to generate sufficient contact force between puck and blade. (Note that a_c = v²/R, where R is the radius of curvature along the trajectory).

(2) At the same time, he must orient the blade so that the contact side faces the center of curvature of the trajectory — in other words, it must face in the direction of the red arrows (shown in the picture). This allows the puck to "push" against the blade with enough contact force to avoid falling off due to gravity. This happens by way of the friction force between blade and puck. The friction force is proportional to the contact force. So a high enough contact force generates enough friction force to counteract the force of gravity pulling down on the puck.

The lacrosse style goal is one subject in the physics of hockey that must be seen to be fully understood. To see this goal made by Mike Legg watch the video below.

Physics in Kite Flying

Kite Flying

Kite flying is a fun activity which people of all ages can enjoy. All you have to do is go somewhere windy and you can literally go fly a kite.

The physics of how a kite gains lift is very similar to how an airplane gains lift. The wings generate lift force by the action of the moving air over the wing surface. A kite works in the same way. The wind blows in the direction of the kite and somewhat underneath it. This creates lift. The figure below illustrates this.

As shown, a string is attached to the kite in different locations so that the kite doesn't flop around in the wind. For further stability (as well as aesthetic value) a tail is often added to the back of the kite. If the wind were to blow the tail from the side, the kite would rotate until the tail (and kite) lined up with the wind. This allows the kite to remain straight and point in the direction of the wind. The photo below shows the tails of several kites.

As shown in the second figure, a lift force is generated in the direction perpendicular to the wind, and a drag force is generated in the direction parallel to the wind. It's the same principal as if you were to stick your hand out the window of a moving vehicle. With your hand tilted clockwise the wind force would push your hand up (due to lift) and back (due to drag). Both lift and drag are unavoidable consequences of the aerodynamics involved. You cannot have one without the other.

Since the lift force acting on a kite is usually quite small, they must be made of very light and rigid material to get airborne and stay in one piece.

To get a kite airborne it is sometimes necessary to run while pulling the kite behind you. This creates "apparent wind" which creates lift and pushes the kite up. Once the kite reaches a high enough altitude where the wind becomes strong enough, you can stop running and the kite will remain aloft.

Physics of Soccer

Soccer

The Physics Of Soccer — The Magnus Effect

When a soccer player kicks a ball off-center it causes the ball to spin. The direction and speed of the spin will determine how much the ball curves during flight. It's the same principle as a curve ball in baseball. When throwing the ball, the pitcher imparts a fast spin which causes the ball to curve during flight.

The curve of the ball during flight is known as the Magnus effect. See the figure below.

Source: Wikipedia via Gang65

As the ball spins, friction between the ball and air causes the air to react to the direction of spin of the ball.

As the ball undergoes top-spin (shown as clockwise rotation in the figure), it causes the velocity of the air around the top half of the ball to become less than the air velocity around the bottom half of the ball. This is because the tangential velocity of the ball in the top half acts in the opposite direction to the airflow, and the tangential velocity of the ball in the bottom half acts in the same direction as the airflow. In the figure shown, the airflow is in the leftward direction, relative to the ball.

Since the (resultant) air speed around the top half of the ball is less than the air speed around the bottom half of the ball, the pressure is greater on the top of the ball. This causes a net downward force (F) to act on the ball. This is due to Bernoulli's principle which states that when air velocity decreases, air pressure increases (and vice-versa).

Therefore, when a soccer player kicks the ball right of center the ball spins counter-clockwise and the force acts left, causing the ball to curve left. When the ball is kicked left of center the ball spins clockwise and the force acts right, causing the ball to curve right. This can result in a ball deviating as much as several feet from the original trajectory by the time it reaches the net. This is no doubt a useful strategy when attempting to make a goal, since it makes the path of the ball less predictable to the goalie as he's preparing to block the shot.

The Physics Of Cheerleading

CHEERLEADING

One of the main principles behind the physics of cheerleading is setting up good weight distribution in the stunts, such as pyramids. The pyramids are created by putting a greater number of people at the bottom than at the top. The stronger (and usually heavier) team members form the support base at the bottom while the lighter team members are at the top. This enables greater ease in performing stunts that involve holding the chearleaders up (at the top of the pyramid), and tossing them into the air.

Stunts, like the ones shown in the three pictures below, rely heavily on having the right number of people at the top and bottom of the pyramid. As you can see, keeping balance is critical during the stunts, as well as keeping an even weight distribution among the team members involved in supporting the weight of the other team members. Even weight distribution is important because it helps the supporters keep the cheerleaders at the top of the pyramid level. If the weight distribution is uneven, one side of the cheerleaders at the top of the pyramid will dip more than the other side, and it won't look as good.

The picture above is a great example of how the weight is distributed evenly among the team members. If you look closely at the picture, you'll see that the two cheerleaders in the middle of the pyramid are each supporting half the cheerleader's weight at the top. And at the bottom of the pyramid there are six team members, all supporting the three girls in the middle and top of the pyramid. Therefore, each team member at the bottom is holding approximately half a cheerleader's weight. Therefore, the weight is distributed evenly among all the team members who are involved in supporting the weight. It's clear that good weight distribution (and how to achieve it), is one of the main areas of focus in the physics of cheerleading.

Some of the more advanced stunts involve tossing the cheerleaders into the air, and them doing flips and twists while airborne. This can incorporate some gymnastics skills as well, which adds a dynamic component to the physics of cheerleading. The figure below shows a stunt in which the cheerleaders are thrown up and into a twist.

The landing of the cheerleaders is cushioned by the support base, which catches the cheerleaders in a cradle position. They do this by bending their knees during the "catch", which allows them to more easily absorb the force of landing. This in turn reduces the forces acting on the cheerleaders during landing.

This concludes the discussion on cheerleading physics.