|MadSci Network: Physics|
As with all products that involve human interaction (such as cars, clothing, computer interfaces, chairs, you name it), the design and engineering involved must take several "needs" into account. These include functionality, performance, reliability, cost, recyclability, safety, compatibility, aesthetics, ergonomics, user-friendliness, and life span (as all so eloquently put by one of my Mechanical Engineering professors). However, many of these needs compete with each other, so every piece of engineering and design comes with compromise. What is more durable is generally more costly, or perhaps does not perform as well. It also may not be as comfortable. You can look at cars and see this very clearly...high performance race cars are less comfortable because they must be very rigid and light, and the priority is given to performance parts such as large engines. There are a few luxury cars which are high performance, but the level of power required to overcome the excessive weight caused by all of the luxury accessories pushes the cost up very, very high. With shoes, it is no different. Running shoes place high importance on performance and functionality. Ergonomics are also pretty high on the list, but aesthetics are not quite as important, maybe. The overall struggle that you'll probably encounter in running shoes is the balance between performance, ergonomics, life span, reliability, and compatibility (due to different foot types). A big struggle with shoes is overcoming the different foot types. A neutral foot is easy to deal with, but what about someone with an over-pronated footstrike (foot rolls in) or a supinated foostrike (foot rolls outward)? A neutral runner can get the best performance because his shoe does not require a lot of additional support. Therefore, the shoe can be lighter, more flexible, etc. A pronated foot will require a lot of additional support, and therefore the shoes will be less flexible. Concerning the more PHYSICS aspect of this, we're going to look at a neutral foot for ease. Running shoes are differently constructed based on their purpose. A training shoe for long distance runners will be considerably more durable and "cushy" than a racing shoe. A long distance shoe or training shoe needs to protect the runner from impact, since impact can damage the joints and is painful. The midsole will probably be pretty soft, with a heel that is more resistant to compression and the forefoot area being even MORE resistant to compression than the heel area. Also, the heel and forefoot areas would be made to absorb shock more than the center of the foot. This makes sense, because the center of the foot (arch) does not take very much impact. The heel and forefoot areas take higher impacts, and therefore must be made to take those impacts. Furthermore, they also need to be resistant to compression (particularly the forefoot) because if the midsole compresses a lot, the runner will be wasting a lot of energy in compressing the midsole rather than putting that energy into motion. It would be like running in sand! However, it would be a bad idea for the manufacturers to put this thicker and more resilient material down the length of the whole shoe, since it will most likely weigh more and be more costly. These things have to be taken into account. A racing shoe will have far, far less padding and will be constructed to be as light as possible. Less energy is wasted in lifting the shoe, less energy is wasted in the compression of padding, and the shoe will be faster. However, the shoe will not be as durable and will leave the joints prone to pain and suffering over long term use. Concerning grip and friction, the bottom of a running shoe may be flat for a track surface or spiked for a grassy surface. The point here is to keep the shoe from slipping on the ground. Energy = force x distance, so if your foot slips, you've lost energy through that distance of slipping. Congratulations! You've warmed the ground by turning your kinetic energy into thermal energy, but that wasn't what you were after...you wanted to move. A flat shoe is good for the track because it provides a full surface area contact and will not have additional weight/material on the outsole. A spiked shoe is good for the grass because once the spike drives into the ground, there are not only frictional forces involved but also normal forces between the spike's surface and the dirt pushing against the spikes. Normal forces tend to be a lot more effective than frictional forces in stopping movement. Also concerning grip, the INSIDE of the shoe would provide better performance, perhaps, if the lining were a high-grip rubber. However, this would tear and blister the skin, so the shoes are designed with a slicker interior (and with the intention that you will wear socks). The foot is held in place by the normal forces on the heel and a combination of frictional and normal forces on the top of the foot. The heel counter is the stiff piece behind the heel that provides stability, but it also surrounds the heel to help keep it in place. Tightly lacing the shoes helps hold them in place by increasing the normal force on the top and sides of the foot. Since the friction coefficient of the material inside is not changing, the higher normal force will cause an increase in the maximum static friction force and will help keep the foot from slipping. Again, any slipping will result in a loss of energy to heat, and that is not the goal of a running shoe. In the purest sense, the most efficient shoe will be as light as possible, not compress at all, and will not slip at all on the ground. However, compromises must be made to accommodate the requirements of the human body concerning comfort. Of course, marketing requires that it look good, too, but I, as an engineer, do not deal with aesthetics so much, so you will have to look elsewhere for advice on that. Hope this helps you some.... Joel Chapman shoe information: http://www.runningwarehouse.com/LearningCenter/ (the best shoe-specific information I could find) concerning friction: Hibbeler, RC. "Engineering Mechanics: Statics" Tenth Edition 2004, Chapter 8.
Try the links in the MadSci Library for more information on Physics.