The Physics of Sports Introductions Names, jobs at Fermilab, what we do. Keep it simple. We’re here to talk about the physics of sports. Engage audience. “Who knows who this is a picture of?”

Physics explains things that are very, very small. Physics studies things that are very, very small. At the smallest scale, we call this elementary particle physics.

Fermilab Accelerator Complex Main Injector, 2 miles around Tevatron, 4 miles around First a word from our sponsor. We work at Fermilab. Who has visited? Who has parents or neighbors working at Fermilab. Who knows what it is? Quick, simple description or facility and research.

Physics explains things that are very, very large. Physics studies things that are very, very large. This is astrophysics and cosmology

Physics explains things that are right in front of us. Physics explains things that are right in front of us, and that’s what we are going to talk about today. Who likes sports? Who is watching the Olympics on TV? How many of you are ice skaters? How many play softball or baseball? Football? Soccer? Basketball? Tennis? Gymnastics? Track? Dancing? Bicycling? Lacrosse? Et cetera… We can describe and explain the motion of objects and their interactions using physics. Understanding what is going on lets us perform better and to design new equipment. First, let’s look at some of the basic rules of physics.

Newton’s First Law Objects at rest remain at rest Objects in motion remain in motion UNTIL YOU APPLY A FORCE Who knows what Newton’s First Law of Motion is? Demonstrations (space permitting): A soccer ball on the floor remains at rest until kicked. A Wiffle ball remains in motion. A ball is pitched to a presenter who swings and misses. This can be one presenter to another, teacher to presenter, or student to presenter.

What forces are important in sports? Newton’s Second Law F = ma What forces are important in sports? Who knows Newton’s Second Law? Quickly – What is Force? Mass? Acceleration? Engage audience: What forces are important? Contact Gravity Friction Air resistance Hold up baseball and softball. If hit with the same force, which will go farther?

Flight Path Motion and forces have a direction as well as a strength. We call those vectors and draw arrows to show them. Here Michael Jordan is shooting a free throw. The arrow shows the initial direction of the ball, which is the direction he applied a force. We can break that down into a part that is straight up and a part that is horizontal. Once the ball is in the air, the most important force on the ball is gravity, counteracting the initial vertical motion and pulling the ball downward down. Gravity does not affect the horizontal motion. So, the ball travels on an arc and swishes though the basket. Here the ball is moving forward with LeBron James as he jumps, and he just has to lift it up and drop it in the basket, not push it forward.

The Home Run Swing The optimum home run angle! Ball arrives on 100 downward trajectory Andre Dawson and Frank Thomas swing up at 250 Ball takes off at 350 The optimum home run angle! Courtesy of A. Nathan, University of Illinois. http://www.npl.uiuc.edu/~a-nathan/pob/

For every action there is an equal and opposite reaction. Newton’s Third Law For every action there is an equal and opposite reaction. Who knows Newton’s Third Law? Do skateboard demo here – Ollie – Skater puts weight on rear foot at tail of skateboard Force of floor on rear wheel holds it in place Downward force of gravity from skater on tail of board drives it down Rest of board pivots up – a lever – center of mass moving upward Tail of board hits floor, skater pushes off and retracts rear leg Board center of mass continues to rise, but slowing due to gravity Skater extends forward leg, retracts rear leg, eventually leveling board All land gracefully together Thank the volunteer. Dribble basketball – ball recoils, but so does earth

Conservation of Energy Potential Energy Kinetic Energy Kinetic Energy Another example of an object contacting the floor Let’s look at it differently We call the principle conservation of energy That means that the total energy of a system remains constant It can change form, but the total remains the same Chemical energy store in the body can move muscles to give the ball potential energy The ball falls and the potential energy turns into kinetic energy, the energy of motion The ball hits the floor and is compressed, like a spring, and energy is stored in the ball The ball bounces back and the spring energy turns into kinetic energy The kinetic energy turns in potential energy as the ball rises and slows down. But it does come back to the same height. It doesn’t have as much potential energy as when I dropped it. Where did that energy go? Heating ball, heating floor, sound, heating air Compression Energy Heat and Sound Energy

Conservation of Momentum Demonstrate: Place a volleyball on the floor. Roll or kick another volleyball at it to hit head-on. What is different if balls are different masses? Driving a golf ball. Club mass ~200 g. Ball mass ~46 g. Club speed ~ 108 mph. Ball speed ~156 mph. Not all momentum is transferred. Club continues to move forward, but slowed. Want peak club speed on contact, so can’t slow down. Must follow through. Energy transfer is force x distance. Longer contact time, better energy transfer. Gravitational potential energy of club head is minimal compared to kinetic energy from golfer’s muscles. In colloquial usage, “having momentum” is equivalent to Newton’s First Law of Motion. A moving object tends to keep moving. If m1 = m2 , then v1 = v2

Equipment features Ball material, size, shape, and texture Properties of the club, bat, racquet, or stick Lots of choices. Engage audience with questions, or at least challenge them to think about why balls, etc. are different Basketball vs. soccer ball vs. volleyball – surface, inflation, durability Tennis racquet vs. badminton racquet Baseball – softball – Whiffle ball Baseball vs. tennis ball – What happens when you hit one?

How fast can you throw a ball? How fast can you throw a ball – major league pitcher – 100 mph max Extend the length of your arm with a lacrosse stick, and you can throw faster – MLL 108 mph Jai Alai – 188 mph

Momentum transferred Energy stored and released Tennis serve – similar extension, but also store energy in ball, racquet and release – Venus Williams 128 mph Driving a golf ball. Club mass ~200 g. Ball mass ~46 g. Club speed ~ 108 mph. Ball speed ~156 mph. Not all momentum is transferred. Club continues to move forward, but slowed. Want peak club speed on contact, so can’t slow down. Must follow through. Energy transfer is force x distance. Longer contact time, better energy transfer. Gravitational potential energy of club head is minimal compared to kinetic energy from golfer’s muscles. David Rawlings, PGA Golf Professional http://golflessonslondon.wordpress.com/

Hockey Slap Shot Another example of storing energy in a stick to get more speed – hockey stick hits ice behind the puck, bends storing energy, energy is released to puck as the blade whips even faster than the lever arm. Compare: mass of puck 156-170 g, mass of golf ball 46 g; speed of puck 120 mph, speed of golf ball 185 mph (Tiger Woods). How much force to break a hockey stick? 2000 pounds.

Pole Vault Progress Another example of storing and releasing energy in a piece or equipment Change of technology The graph is the height of the men’s gold medal jump in the Olympics. Until the 1950’s pole vault poles were made of bamboo. Techniques and training led to slow improvements in performance. In the 1950’s composite poles were introduced and finally legalized. Stronger, engineered, uniform. Performance jumped. Needed big improvements in landing pits – sawdust/sand -> foam, air cushion Good example of using a graph to see an effect http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/Daniel_Lenord/vault.html

Air Resistance (drag) What do these athletes have in common? Trying to go fast – reduce drag Reducing drag by wearing tight-fitting, slippery clothes Streamline position Have you every ridden your bicycle with a big billowing coat? What did it feel like? Swim suits mimic texture of shark shrink for efficient glide through water, compress body for streamlined position. So good that some have been banned from competition. What is “fair”?

F R I C T I O N Friction can be good, even essential. Notice the chalk on the hands to absorb sweat and give good grip.

Sometimes you want some grip, but not too much Sometimes you want some grip, but not too much. In gymnastics you need to be able to slide in a controlled way. The need varies from event to event. If even a miniscule fraction of the kinetic energy in the gymnast’s body goes into irritating the skin cells, she or he can get nasty blisters.

Resistance & Friction In running you need friction between your foot and the ground. Different shoes. What sport is each for? Why? In sprints, you want spikes. For initial acceleration at start, blocks to push against. Usually you want to reduce your air resistance, but for training you want extra resistance. Some places you can run up hills. Some people use a small parachute to provide extra resistance. Demonstration if there is space, time: Two volunteers – self-identified as good runners. Race over a fixed distance Outfit the winner with the parachute and race again Ask him/her to describe the feeling. Thank the volunteers.

The Power of Spin The kick from a crouch just uses the energy from a short kick. Jet Li builds energy and momentum with several steps. This shows that there is energy in a spinning body. Fancy name – “Conservation of angular momentum” Show bicycle wheel as gyroscope – doesn’t fall over A well-thrown pass in football uses the same idea. With time, two student volunteers can throw the football back and forth. Thank volunteers. The ball spinning on its long axis stays pointed in the same direction. Compared to a tumbling ball, it has less air resistance, so on a long bomb it can travel five or ten yards farther. And it’s easier to catch.

Spins in Figure Skating You can change how fast you are spinning. Which skater is spinning fastest? How can you tell? Watch the skater jumping. Arms spread on approach, arms in to spin, arms out to land. Same thing in dancing, gymnastics, diving Demonstration 1: Volunteer – thin with long arms is best Explain to volunteer what will happen. “If you start to lose your balance, put your arms out.” Volunteer stands on turn table, holds weights in hands with arms half way extended. Stand behind, place hands on volunteer’s shoulders, and give a gentle twist. Instruct to put arms out. Volunteer slows. Instruct to pull in tight. Volunteer speeds up. Arms out. Arms in. Arms out. Thank you. Demonstration 2: Volunteer – thin with long arms is essential Volunteer stands on turn table and holds bicycle wheel in front, away from body. Caution volunteer that wheel will be spinning fast and it is important not to let it touch body. Steady wheel with one hand, spin it up with other hand. Instruct volunteer to twist wheel to one side (up to 90º). Volunteer turns. Instruct volunteer to twist wheel back the other way. (They often do so spontaneously.) With wheel straight up, take wheel (still spinning) from volunteer very carefully and slow it down. Thank volunteer. This is the way snowboards, divers, gymnasts change their twists and spins in the middle of the air.

Reaction Time Reaction time is important in most sports. A ball or puck may be coming at you at 60, 70, 90, 120 miles per hour or more. You may be traveling that fast and need to react to conditions.

Downhill skier speed: 80 mph Olympics. Crashes. Only in fastest races are the skiers allowed to even see the course before hand. Downhill skier speed: 80 mph

Reaction Time in Baseball What is a millisecond? 100 ms to see ball 75 ms to process information 50 ms to decide whether and how to swing 25 ms to start swing 150 ms swing 400 ms total 450 ms available at 90 mph ±7 ms ball goes foul Courtesy of R. Adair through A. Nathan.

Measure your reaction time… Let’s measure your reaction time.

Compare your reaction time… Volunteer(s). Presenter will drop ruler and volunteer will grab it with thumb and index finger. The distance it falls can then be converted to time. In a classroom and with enough rulers, the students can break into groups of two or three and everyone gets a chance. In a larger group, bring one or two volunteers to front and each presenter can drop the ruler for one volunteer. Or just do one. In small group, distribute rulers and handouts. Dropper holds ruler vertically with 30 cm mark at top. Grabber puts thumb and index finger on either side of ruler even with 0 cm mark. Dropper raises or lowers to align. If possible, grabber should rest forearm on a table or desk to prevent motion of arm. “I’m going to drop the ruler and you are going to grab it. How far it falls will measure your reaction time. OK?” Grabber practices closing fingers quickly a few times. Dropper warns. “Are you ready?” Drops. Read how many cm the ruler has fallen. Look up the time on the chart. Repeat once or twice. Compare complexity of deciding to squeeze fingers with complexity of analyzing where a ball is coming, how fast, which way it is curving. Or how to approach a slalom gate or adjust a bobsled. Give teacher handouts for later use. Equation used: t = √((2d)/g)

Physical laws explain the world Newton’s Laws of Motion Conservation of Energy Conservation of Momentum These laws help us explain everything about sports, but there is more to learn. Physics really is everywhere!

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The Physics of Sports