# IVIR

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1. ## Jenga

This past weekend, I saw a giant game of Jenga at MIT. Literally. The blocks were nearly 2x4s, and the structure was taller than I am. While I did not stay to watch, it is interesting to think about a few of the different strategies that I remember from my childhood days. First of all, I used to believe that the faster you pulled the object out, the less chance a collapse would occur. While I'm not sure of my logic behind this reasoning, I most likely imagined that hopefully the structure just wouldn't have time to collapse if I pulled it fast enough (Yeah, I know). However, after the block is removed, whether quick or slow, the structure will still have the exact same properties regardless of speed. Another theory may be to reduce friction, but it is important to note that the frictional force does not rely on velocity, it relies on the normal force. The one factor that does effect the result of the turn is how straight you are able to pull the block out. By pulling the block straight out, you are minimizing the normal force, but if you tilt to one side or another, you are increasing the normal force and creating a larger frictional force. Another concept of the game Jenga is torque. Since torque is F x r and the r in most jenga games is relatively small, the structure can often withstand the removal of blocks that may have seemed impossible. The middle block is at the center of the fulcrum, so the r would be 0, allowing players to theoretically remove all of the outside blocks while keeping a cross pattern in the middle. This is much easier said than done due to the friction caused by uneven pulls (an even perfect pulls as the wood has a large surface area) and the fact that even a small breeze can cause enough torque in the other direction to knock the tower down. A horizontal breeze may have a small force, but since the center point is technically the ground in this plane, the r would be as tall as the tower. Hopefully, the physics of Jenga could help people improve their gameplay, but to be honest, isn't the best part watching it all fall?
2. ## Horsepower

Cars, especially sports cars, often have their horsepower compared as a somewhat ignorant method to determining the faster car. However, this is not the most accurate way of making judgement because there are other variables, some much more important, in determining a cars acceleration or top speed. While an engine's horsepower provides the force, accelerating the car, it is important to remember the basic fact that F=ma. Therefore, a 4000 lb car with twice as much horsepower as a 2000 lb car will accelerate at the same speed. This is the reason that lightweight sports cars that are often half the price of supercars have similar accelerations. Especially with the Porsche 911 GT3, any possible excess weight is removed to allow for the greatest acceleration, one that can compete with Lamborghinis and Ferraris. Also, when people assume that more horsepower is equivalent to a greater top speed, they are ignoring a very important concept: terminal velocity. Yes, it is important to have a large enough engine to accelerate the car at a great rate, but ultimately when you start getting into the 200 mph area, design is much more important than power. As we learned earlier this year, air resistance is directly related to velocity. However, the extent of the air resistance varies greatly depending on the shape of an object (For example, dropping a book vs a sheet of paper). Therefore, it is necessary for car designers to minimize air resistance while still creating downforce in order to create the best results. The balance of these two goals and new advances in materials/aerodynamics is the reason that street legal cars have been increasing in performance greatly, but also the reason that creators are having a hard time creating a road legal car that hits above 250 mph, while still looking/feeling comfortable.
3. ## Second Bounce

Have you ever dropped a ball and had it bounce once normally before taking a crazy second bounce? I'm always watching out for this phenomenon when I have a lacrosse ball on a hard surface, but I've never really understood what was going in. The main factor causing the crazy second bounce is actually the spin on the ball acquired during the first bounce. As the ball falls, it usually has a small amount of spin or is traveling at an angle that isn't 90 degrees. As the ball hits the ground, the ideal,perfectly elastic collision does not happen as there is friction between the ball and the ground. This friction is responsible for creating a backwards force of the surface of the ball, resulting in a much greater spin than before. When the ball then hits the ground the second time, the spin takes effect as the ball shoots off in the direction of the spin, reaching a low vertical displacement but far horizontal displacement. Because of the role friction plays in adding the additional spin on the first bounce, this effect is not as prevalent with balls that do not have high coefficients of friction with other surfaces. Since rubber does have a pretty high coefficient of friction with other surfaces, lacrosse balls and most bouncy balls follow this abnormal pattern, causing disaster for any ignorant owner who happens to drop it.
4. ## Real Time Physics---- Madden

In the past few years, Madden NFL video games have adopted real time physics, an extremely complicated technology that allows the players in the game to react according the actual physics. In previous Maddens, a spin move would result in one of four tackles, and there were only about 10 different types of tackles, making the end of plays appear too staged. However, real time physics enables the characters in the video game to react to a force from an opposing player as they would in real life, causing an infinite number of tackles. For example, if a linebacker hits the running back with the "hit stick", the running back may fall down immediately, stumble for a few steps before falling, or even regain balance and keep running. Real time physics is also used for fluency in movements as making a catch no longer seems robotic. While Madden may be incorporating real time physics into their game play, I've started to think that the NFL enjoys testing the limits of physics with crazy coincidences and plays. For example, the front flip touchdown seen below, is a classic example of angular momentum along with torque. The player was relatively stretched out with the intent on diving into the endzone, but the defender hit his feet. Even though the force was not great, the long distance of the lever arm to his center of mass created a high torque, causing the runner to tuck their body in, now realizing that they are going to complete the revolution. And all of this happened in a few seconds.
5. ## Frisbee

The simple art of Kan Jam or any other frisbee related game seems skills based, but it may help to have knowledge on the physics of the flight as well. The first aspect of a frisbee is the design. Most frisbees are relatively thin with curved edges, which make it more aerodynamic, creating lift while in flight. Similarly to cars, the curved shape at the front of the frisbee allows for air to pass over the frisbee faster than air passing under it, making the air above the frisbee at a lower pressure than the air below the frisbee (aka lift). Also, it is obviously important to make the frisbee spin in the air in order to create stability by increasing angular momentum that the simple force of air resistance has a hard time overcoming. Therefore, the more spin you put into your throw, the more stable its flight will be. This is perhaps the most useful piece of information as newcomers to Kan Jam often toss the frisbee gently, which only results in the frisbee sliding to one side or the other. In ultimate frisbee, there is a "kickoff" type of event in which one team tosses the disk as far as possible (or so it seems). On a single throw for distance, it would appear to be extremely beneficial to try to optimize the launch angle in order to provide a proper amount of lift without too much drag, but also achieve a sustained projectile motion flight with a large x component. Here's some frisbee trick shots for fun
6. ## Shape and Dolphins

It is not surprising that shape has a lot to do with the movement and speed of objects, but it may be surprising that dolphins pose such a question to researchers when it comes to their advanced swimming techniques. The shape of an object can determine numerous factors of an object that are integral to its performance. For example, race-cars are built with a specific shape in order to produce high amounts of downforce while airplaines are built with their wings in order to create lift when piercing through the air. Downforce is produced by channeling air above a specific shape, causing the air resistance to have a small impact on overall speed, but have a significant impact on providing a force down on the object. Lift is the opposite, as airplanes are designed to channel air underneath the wings in order to keep the plane steady in the air, as gravity and the generated lift can hopefully come close to canceling each other out. Since air resistance is related to the velocity of the object, lift and downforce rely on the velocity of an object as well. The faster an object is traveling, the more air being channeled over/under the object and the higher the resulting force. This is why it is even potentially dangerous to go at lower speeds in an airplane or F1 racecar. Dolphins travel quite fast through the water compared to their complexion, making researchers wonder why they are such fast swimmers. Although not all of the answers have been found, it has been discovered that dolphins reduce drag throughout the water by shedding their outer layer of skin every few hours. Again, people are still not sure why this process would be beneficial, but it is quite cool nonetheless.Another important aspect of a dolphin's complexion is its tail fin. This fin is slightly curved in order to provide a greater thrust than a flat fin when stroking through the water, helping the dolphin obtain high speeds. It is pretty cool that dolphins pose such a big mystery to physicists and biologists alike as they swim more efficiently than what seems possible, making them able to stump humans.
7. ## BMX

As usual, I found myself watching quite a bit of YouTube over break, especially different BMX videos. While I could only dream of pulling off the tricks the professionals do, watching it makes me feel nervous as I realize the potential consequences of a nasty fall. I have kind of a lot of blogs on landings/falling, so I figure I will use this blog to tackle one of the most crazy things a bmx rider will do: grind a downward sloping railing on one peg. For those not extremely comfortable with BMX biking, pegs are a small metal attachment that can go on either side of the front and back wheels. BMX riders actually jump onto a railing, balance on one peg for a short amount of time before landing comfortably. They make it look easy, but there are numerous factors at play. First of all, the rider must have a clean takeoff in order to hit the railing in the desired location. Secondly, the rider must withstand the impact of one peg landing on the railing without causing the bike to rotate. Since a single peg is on a single side of the bike, landing on one peg creates a torque as one corner of the bike is on a stable surface while the other side is still in downward motion. It is up to the rider to shift their weight at the point of impact in order to offset this torque and remain balanced. Another component of the maneuver is to remain balanced on the rail throughout the entirety of the trick (even though they are not on the rail for long). You will see that riders will turn about 15 degrees from their impact time on the rail to the landing as the horizontal momentum of the back of the bike keeps going, since it does not have something to stop it like the front of the bike has the peg on the railing to stop the motion in that direction. Finally, the landing requires the rider to have a little "give" in his arms and knees, so the impact is not as strong as a force as it could be. Locking the arms or wrists often results in a broken wrist as the human body is not built to withstand large forces in recreational sports. Here's a little video of some cool tricks that I couldn't even start to describe the physics of...
8. ## Turf: the good, the bad, the ugly

Playing sports in high school, you start to look forward to any opportunities to leave the swampy grass for the fresh turf field. Since Pinegrove fields are uneven, rocky and often have large spots of grass missing, the turf is always a better option. However, playing on turf does not always end well for a variety of reasons. The good: traction. Turf provides excellent traction as it increases the coefficient of friction between the ground and cleats, enabling for sharp movements. On grass, the force of pushing off of a foot could cause the soggy grass to move, but turf remains firm, even in the most adverse conditions. Also, grass contains numerous rocky/hard spots, where the force of a step is unable to penetrate the surface of the earth, causing a lack of traction. Without friction between the human and the ground, there is no force propelling the human forward, prohibiting movement. The bad: heat. While this may be a mixture of physics, chemistry and biology (I don't really know), the surface of turf heats up to extreme temperatures in the heat. One reason for this is that the tiny rubber pellets absorb more energy than they release during the day (when it's sunny), but also the fact that artificial grass cannot evaporate like normal grass. Evaporation causes cooling as it is an endothermic process, making its surroundings appear cooler. The heat may feel good if you lay down on the turf in April/May, but when I went to Maryland for some lacrosse tournaments in July, people's cleats were literally melting. The ugly: turf burn. Friction is always switching between good and bad, but necessary for life. We want less friction when we kick a soccer ball to make it roll further, but at the same time it wouldn't roll without some friction. While the friction on turf is a blessing for running, it becomes problematic when you end up on the ground in a game. Although I am personally unaware of the exact number, I can attest to the fact that the coefficient of friction between the turf and one's skin is extremely high. The high frictional force, dependent on the subject and his/her actions, can even demolish large amounts of skin, causing anything from a red mark to an open wound. Overall, I would take the average turf field over the average grass field any day, but nothing beats a perfectly groomed grass field on a sunny day, as it is safe and efficient.
9. ## Boxing vs UFC

Boxing and UFC are both entertaining (when there are available without the astronomical pay-per-view prices), but the physics of each one is pretty different. If anything, UFC is a more dangerous sport, making boxing look like a children's show. The major difference is due to the different types of gloves used in each one. Boxing gloves are pretty well known, but I've included pictures of both types below to make the comparison easier. While boxing gloves have quite a bit of padding, UFC gloves barely cover one's hand. As we have learned in physics, padding makes quite a difference on the force felt by an object. Since the change in momentum of an object equals force x time of impact, it is obvious that a longer time of impact will lower the force felt by the individual. In the case of boxing, the opponents arm has the momentum, which is being changed to 0 by your face (hopefully just your body, but you never know). The padding from the glove makes the entire impact take longer, therefore decreasing the force felt by your face. In UFC, the smaller amount of padding means that your face feels a higher force. There is another big difference between UFC and boxing; you can kick in UFC. Sure, a punch by Floyd Mayweather may still knock you out, but kicking can provide even more force. Also, it is important to remember that no glove is worn on the foot, so any impact from kicking is not padded at all, increasing the force felt by the recipient. Most people's legs are longer than their arms, so people can generate much more torque with their legs than their arms, creating powerful kicks. I'm willing to bet the angular momentum and torque generated by a roundhouse kick will nearly knock somebody unconscious, but this is one experiment I wouldn't want to test myself.
10. ## Tire Swing

Growing up, my best friend had a tire swing in his backyard. While it provided trivial entertainment, looking back, a tire swing involves quite a bit of physics. First of all, there is the connection to the tree. The swing needs to be far enough away from the base of the tree in order to prevent accidental collisions, but it also needs to be sturdy enough to withstand human weight. The further out from the base of the tree you put the swing, the more torque that is applied to the branch as the lever arm is being increased. Perhaps this is why they have the "do not try this at home" warning on most innocent projects. Another aspect of the tire swing is the ideal motion of the swing, similar to that of a pendulum. Like a pendulum, the rope is nearly mass-less in comparison to the tire and human object, allowing for simple harmonic motion. However, air resistance and other movements prevent the pendulum from reaching its previous height each time, creating a dampening effect, even though it is extremely slow. In addition to this motion, the tire swing often rotates. Similar to the common figure skater example, a stretched out human will rotate slower than a balled up human, due to the indirect relationship between rotational inertia and angular velocity as they are multiplied to equal the consistent angular momentum. Finally, the tire swing usually ends up following an ovular or circular path instead of a straight line, so it would be interesting to study the exact physics of a pendulum in a non-linear path. It is also important to remember to hold firmly onto the swing, as a fall from the swing could result in serious injuries (Witnessed it firsthand) due to the acceleration of gravity, but also unforgiving nature of the ground, specifically tree roots. The hard surface of the roots keeps a low impact time, causing a high force to be felt by the person as their momentum is immediately changed to 0. EDIT: Didn't see Kate's blog on this, sorry kate :/
11. ## Snow and Rain

Since we learn that objects consistently gain velocity in free fall motion due to the acceleration of gravity at 10 m/s^2, why doesn't rain and snow wreck havoc since it is falling from an insanely high distance? One reason for this lack of speed, especially with snowfall, is because of air resistance and drag forces. The net force on a snowflake would be the weight (mg) minus the drag forces acting on that object. Since snowflakes are relatively porous and non-aerodynamically shaped, the drag force is relatively large. Perhaps more importantly, a snowflake barely has any mass so the force of its weight is barely noticeable. Also, by the time the snow, or rain, reaches a height where we can see it, the snowflake or raindrop has reached its terminal velocity, which is not very high. Rain does indeed travel faster than most snow due to less drag forces, but there is a specific reason that rain does not hurt us instead of simply plopping onto our skin. Rain is a liquid, so when the raindrop does hit our body, the force isn't completely transferred to our body as the water droplet splatters itself. However, large bodies of water, like a large can still exert large amounts of force without absorbing a lot of the impact force. For example, if you cliff jump from an extremely high distance into water, the force of impact on the water is still extremely high. Part of this is due to relatively high surface tension of water, but the other part is that the water cannot dissipate all of the force of impact since it is a liquid, not a gas. These same reasons are also the reasons why hail and ice storms can cause serious damage as they are significantly more massive, and their shape/composition allows for lower drag forces. Large pieces of hail still reach a terminal velocity, but this terminal velocity along with its relatively large mass allow it to create immense damage, including puncturing car windshields. The car windshield and hail are in contact for such a little time that the impulse force felt by the windshield is incredibly large. Also for a follow up on my previous blog about bulletproof glass, a car windshield is layered with plastic as well, preventing it from shattering upon impact. This is also why bullet's penetrate car windows without shattering them for the most part. Rain and snow cause a ton of damage across the world each year, but on the bright side, there is no need to run for cover when these infamous particles are failing from the sky. If they are large hail particles though, put your car in the garage, cover your house with steel and get inside because it is no joke.