A Practical Application of the Fundamentals of Physics.

 

Gravity, the force that attracts a body toward the center of the Earth, seems to be out to get me. I have been described as being “made out of fall down”. This is because I fall down. A lot. I have long legs and big feet and sometimes I don’t pick them up, so I trip. I ride my bike to work a lot and sometimes the potholes get me. Occasionally my adventures in science result in mystery bruises. Bruises and scrapes I can handle, but recently I had the opportunity to test some of Newton’s Laws in other ways.

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I, in my little Dodge Caliber, was hit by a GMC pickup truck. After I took a hot minute to get my wits about me, I crawled out and looked at was left of the tail end of my car. My first thought? “Good job, crumple zones. Good job….” This is how we got to this blog entry. It’s been a while since High School Physics, so let’s all get caught up on some basics:

  • Inertia is the tendency of an object to resist any change in its velocity (speed+direction).
  • A fancier way to say that? Newton’s First Law of Motion states that a body at rest remains at rest unless acted upon by an external force and a body in motion continues to move at a constant speed in a straight line unless it is acted upon by an external force.
  • Force = Mass x Acceleration (if Acceleration is the rate of change of the velocity)

 

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In other words, unless some outside force acts on an object it will keep on going or staying, as the case may be. One of those outside forces is friction. Which brings us to inertia. A bigger, heavier object will take longer to get to a high rate of speed, but if the same force is applied, it will also take a longer time to slow down too. So a ping pong ball takes a lot less effort to stop than a freight train, but it also takes a lot less effort to throw a ping pong ball than it does a freight train. And so that brings us to the practical application portion of today’s blog.

Specifically, in the case of my accident, my little car had almost come to a stop when I was hit from behind. Since the truck was so much bigger, the truck had more momentum than my car brakes could handle—so I was pushed forward, even though the truck slowed significantly.

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Even though there was a lot of damage done to the rear end of my car, I was still safe. This is because some physicists and engineers (thank you!) have been working to make vehicles safer. To do this, they have to take into account Newton’s Laws of Motion. Some of the safety features cars have these days are seat belts, crumple zones, air bags and specialized tires. Since you can’t instantaneously change the mass of the vehicles in an accident, your best bet is to change the acceleration to reduce the force. The function of the seat belts, crumple zones and air bags is to do just that by slowing things down more gradually. They change the acceleration of the person inside the vehicle by increasing the time it takes for the accident to occur – even if it is just by fractions of seconds.

Seatbelts comprise about 50% of your protection in a car. When a driver stops the car suddenly, the driver tends to lunge forward, because the driver’s body tends to maintain its speed and direction. The seat belt holds the driver and prevents the driver from flying forwards when the car stops. Seat belts help by applying a force that overcomes your inertia as in Newton’s First Law. They also increase the time in the wreck which results in a lesser impact force on you; more time means less acceleration to you! Even when your body comes to a stop, however, your internal organs continue to move, slamming against each other because of the impact. So, that’s fun.

Good tires are also an important safety feature on your car. The friction between the tires and the road determines the maximum acceleration and the minimum stopping distance. If the surface of a tire is rougher, then the friction force is larger. This is super important if you are slamming on your brakes to avoid something or speeding up, also to avoid something.

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Prior to 1959, people believed the more rigid the structure, the safer the car. This ended up being deadly because the force from the impact went straight to the passenger. Crumple zones are specially engineered areas on your car that are designed to absorb energy as they are crushed and slow down the rest of the car more gradually. They absorb energy from a collision and therefore reduce the force of a collision on the passengers. They aren’t just spots that are softer or less dense on the car, they are specifically engineered to crush in a relatively gradual and predictable way that absorbs much of the impact energy, keeping it away from the occupants in what is termed a “controlled crush”.

So! Buckle up and be safe, and good job, crumple zones…good job.

Sports Science: Olympics Edition ‒ Javelin

Every four years, the eyes of the world shift towards a global competition, complete with feats of strength, determination, talent and teamwork. The Summer Olympics are back, and I could not be more excited. The following post is one of three about some of my favorite events.

This go-around, I’m rather excited to further study one event in particular. After the 2012 London Olympics, I returned to St. Louis for my junior year of college to find that my roommate decided to join the Washington University track and field team. Why? “I just wanted to be better at throwing spears.”

As he soon learned, Javelin is much more complicated than just throwing a spear. The best javelin throwers have spent decades perfecting their technique. And when I tried to just pick up a javelin and throw it for fun, the result was too embarrassing to share here.

First, there’s the run-up, the series of steps where the thrower is building up speed and momentum. This begins with a traditional run and transitions into crossover steps before the actual throw. The crossover steps are when a thrower swivels his shoulders and hips and orients his body to prepare to release without sacrificing velocity. As a result of the run-up, some athletes can throw the javelin at speeds up to 30 meters per second, or about 100 kilometers per hour!

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As the thrower prepares for the delivery, there has to be a transfer of energy from the body to the javelin itself. This process is started by planting the back foot while still keeping the throwing arm as straight as possible. The front foot continues the transfer by stopping and planting in as straight and rigid a manner as possible. This abrupt stop will force the body to swing around as momentum continues to carry it forward. The athlete must control this swing yet remain rigid in the front foot as much as possible to generate the most force.

Once the front foot plants, a chain reaction of sorts proceeds up the body, transferring force and energy from the legs to the javelin through a series of joints. The back knee swivels forward, followed by the back hip, moving upwards to the shoulder, out to the elbow and finally the javelin.

The javelin is released over the shoulder at an angle ideally between 32 and 36 degrees, according to Mike Barber’s excellent post on the biomechanics of the javelin. As Barber astutely points out, the more important angle measurement is the difference between the angle at which the javelin’s point is moving and the angle at which the javelin’s center of gravity is moving. Ideally, the difference is zero and the pair align to produce the maximum distance. A variation in those angles means the javelin will not travel in the most aerodynamic way possible and, due to increased drag, will not go as far.

Javelin throw at Gay Games 9 Cleveland + Akron

Keep your eye on the motion of the body right at the moment of release of the top javelin throwers. Their arms are straight throughout the run up, and the muscles in their legs have to be strong and robust to handle the abrupt stop required for the best throws. There is a lot of stress put onto their bodies as a result, and they must be in top physical shape.

The javelin throw finals at the Rio Olympics will take place on Aug. 18 for women and Aug. 20 for men.

Sports Science: Olympics Edition ‒ Gymnastics

Every four years, the eyes of the world shift towards a global competition, complete with feats of strength, determination, talent and teamwork. The Summer Olympics are back, and I could not be more excited. The following post is one of three about some of my favorite events.

The spotlight was on Gabby Douglas four years ago in London, as the American led the “Fierce Five” to a Gold Medal in the team gymnastics competition and earned Gold again in the individual all-around competition. This go-around, Douglas has taken a backseat to Simone Biles, a resident of Spring, TX and the favorite to win the individual all-around competition in Rio de Janeiro.

One of Biles’ best events is the balance beam, in which she has won individual Gold Medals at the World Championships the last two years. To earn the most points in this event, the gymnast must risk the most while avoiding a slip or fall.

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Maintaining your balance in everyday life is a case study in the physics concept called center of gravity. The center of gravity of an object is the singular point where all the components of the forces of gravity acting on the object balance out. If you grab a pencil and try to balance it on your finger, the point where it does not fall off either side is its center of gravity. The center of gravity of the human body is around the belly button; as long as the body’s center of gravity is over the person’s feet (or whatever part of the body is touching the ground), the person will remain upright.

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The concept of center of gravity is how tightrope walkers stay on the rope. Usually, a tightrope walker will hold a long stick perpendicular to the rope; this spreads out the total weight and lowers the center of gravity of the person, making it more difficult to be knocked to one side. It’s much more difficult to knock over a bowl than a tall glass. As a result of the lower center of gravity, tightrope walkers have a generally easy time walking across!

However, this trick does not work on the balance beam. Biles and other gymnasts must have complete control of their center of gravity to keep from falling over to one side, and the space they have to balance over is thinner than the width of most people’s feet.

An Olympics regulation balance beam is five meters long but only 10 centimeters wide and held 125 centimeters off the ground. There is very little room for error. Simple maneuvers like turning 180⁰ can go horribly wrong with a slight movement. More complicated maneuvers like handsprings and free aerial cartwheels (both part of Biles’ routine at the 2016 Pacific Rim Championships) require hours of meticulous practice to avoid a fall.

OU Gymnastics at TWU - Beam

To complete those maneuvers, Biles relies on the concept of angular momentum. This is the idea that a spinning object will keep spinning at the same rate unless acted upon by an external force. Basically, for the optimum spin on a handspring or cartwheel, the gymnast must rotate around her center of gravity. If she does so, she will continue the spin all the way round until coming back into contact with the beam.

With some natural talent, hours of practice and the help of some physics, Biles and Team USA is in prime position to bring home Gold Medals galore!

Gymnastics events at the Rio Games begin Aug. 6 and conclude Aug. 16.

Sports Science: Olympics Edition ‒ Freestyle Swimming

Every four years, the eyes of the world shift towards a global competition, complete with feats of strength, determination, talent and teamwork. The Summer Olympics are back, and I could not be more excited. The following post is one of three about some of my favorite events.

As a college student, one of my pre-Finals rituals was to stop studying 30 minutes before leaving for the exam and instead watch sports highlights videos to get pumped up. There was Vince Young coasting into the corner of the endzone in the 2006 Rose Bowl, Tracy McGrady scoring 13 points in 35 seconds to knock off the Spurs, Landon Donovan sending the U.S. to the knockout stages of the 2010 World Cup, and, the grand finale, the 2008 Men’s 4x100m freestyle relay, the race that earned Michael Phelps his second of eight gold medals at the Beijing Olympics.

That race was, to me, the most memorable moment of a historic run for Phelps. Set aside the volcanic eruption of American pride for a second, and just consider the physics at play as anchor leg Jason Lezak breaks French hearts and sends his teammates into delirium.

Mickey Kelly Swims

Freestyle swimming in and of itself is a case study in aerodynamic motion. The swimmer’s body must be in as straight a line as possible while moving through the water to reduce drag. The swimmer’s face needs to be down as much as possible to allow the round waterproof cap the opportunity to part the water most efficiently. Even when the swimmer turns his head to breathe, the horizontal line should be maintained and the deviation in motion should be minimized.

The body should be in a constant state of motion, and all motion should be synchronized as much as possible, with kicks matching the strokes of the arms. Deviations from this synchronization will cause drag. In addition, small, quick kicks are generally more effective than large kicks that require more time; essentially, the sum of many short accelerations is greater than one large acceleration.

So going back to Lezak’s final 50 meters in the relay, it is important to first set the scene: Lezak was trailing the world-record holder in the 100-meter freestyle, France’s Alain Bernard, by about 0.5 seconds heading into the last leg of the race. That lead had expanded to 0.6 seconds after the first 50 meters.

Lezak closed the gap, in part, due to a principle of physics common in racing of all kinds: drafting. The concept here is you get as close as you can to the racer in front of you; that racer absorbs the brunt of the drag, leaving a pocket of “clean” air (or water). The racer behind the leader uses less energy to go the same speed or can use the same energy to gain ground.

Note how Lezak is swimming at the top of his lane, as close as he could possibly be to Bernard’s slipstream. Even though he is not directly behind Bernard, his positioning is actually much more similar to the way that birds fly in a V-shape while migrating. The ideal position for this technique is at around the waist of the other swimmer, which you might notice is just where Lezak is at about the halfway point of the last length of the race. The disturbance that Bernard is causing in the water radiates outwards and creates a small pocket of clean water for Lezak to cut through.

Studies have shown that swimmers in open water races using slipstreams to swim consume about 10% less oxygen than others and reduce the rate of perceived exertion by 21%. And while it’s a technique that annoys people in the neighborhood pool, it’s something that helped Lezak make up a half-body length deficit in 25 meters and win the race by 0.08 seconds. Oh yeah, there’s also that Olympics Gold Medal.

Swimming events at the Rio Games begin Aug. 6 and conclude Aug. 13.