Friction in sports


The purpose of this lab report was to show that friction is involved in sport and can influence decisions on designing and buying footwear. The two experiments based around this report were on the coefficient of Friction and the angle of Friction. Both experiments were carried out in a laboratory at the University of Sunderland. Students from the University undertook the experiments and discovered that the best type of footwear to use for frictional purposes are trainers with thick tread wear on the sole of the shoe. The most frictional type of shoe used was the Adidas Barricade having a coefficient of friction of 0.78. In the second experiment metal was determined to have the highest coefficient of friction out of the surfaces the report used. It showed that metal had a coefficient of friction of 0.84, with the lowest being rubber reporting a score of 0.6.

Introduction
Runners who want to buy a new pair of shoes, footballers who want to have the best football boots to ice skaters who need new ice skates, all have one thing in common. They are seeking a shoe made specifically for them, whether it is to maximise friction or minimise it entirely the choice is based around it. What is friction though? Friction is a force acting parallel to the interface of two surfaces that are in contact during the motion or impending motion of one surface as it moves against another (Hamill 2003). Isaac Newton came up with his first law of motion based around friction. “In the absence of external forces, motion in a straight line and at constant speed continues indefinitely”. For instance when you slide a hockey puck down the ice it will travel on for so long until eventually slowing down and coming to a stop. Newton realised that the only force stopping the puck sliding down the ice was friction in-between the ice and the puck. But if the ice was so smooth and had no friction at all then the puck would be able to travel down the ice until another force was able to stop it (Stern 2004). Which is why with sports performers having the correct footwear on is vital and even the correct hand wear. Which is why in javelin equipment is very important. They need enough friction on their hands to grip the javelin but also to allow it to release smoothly into the air. As well as their footwear, if the shoes they have on will not allow the athlete to stop immediately, then they are either going to injure themselves or go straight past the line and have a foul given for the throw.

Friction is also involved in our bodies. When running, jumping, throwing, walking parts of our anatomy inside are rubbing against one another. Which is why the body has been designed for such forces. Hyaline cartilage is formed around bones which provides a springy pad that absorbs compression at joints (Marieb 2007). Without this then bones will rub vigorously together which will cause pain at the joint and wear down the bone. But not everywhere can stop friction becoming a nuisance. A common injury occurs with runners or cyclists called Iliotibial band friction syndrome. This is when the iliotibial band passes over the lateral femoral epicondyle continuously over a given amount of time. It is caused when the knee is continually flexed of a greater angle of 30 degrees or more, where the iliotibial band crosses over and forces itself on the epicondyle (Ellis 2007). This then causes pain and can result in surgery being needed for it to be healed.

A major area where friction is always thought about is in the work place. Where slips and falls are one of the most common places to happen (Way Li 2007). This is why having the correct footwear on will help to minimise the risks of falling, and is even more important when surfaces become wet and reduce the friction to a small sum (Hammil 2003). When stepping on a wet floor, a shoe sole cannot touch the floor surface without squeezing the liquid out of the contact area. The liquid between the floor and the shoe separates the two contact surfaces and reduces the friction between them. If the correct footwear is not in force then injuries are bound to happen. This then results in what type of shoe to have and what type of tread that shoe needs when working in a practical workplace.

One of the topics talked about in this report will be about the coefficient of friction. The coefficient of friction is a major factor when deciding on what footwear to have.It is the ratio of a force between two bodies and an external force pressing them together (Hamill 2003). The lower the coefficient of friction, the less friction there is between the two surfaces. This is all down to what type of surfaces are in contact with each other. Which is what one of the experiments that will be discussed in this article. Another experiment that will be talked upon is the angle of friciton. This is when fricitonal properties can be measured by, calculating how long the horizontal force and the maximum angle can be before an object starts to slide. The angle of friction is useful to determine which type of shoes can keep itself planted on an object. This is all down to the type of tread that is desgined on the shoe and how it is made onto it. But wanting to overcome friction a lot of energy is lost in the form of heat, and causes the shoes to wear down over time. So having brand new shoes would be best to decide what shoe will have the best angle of friction.

Methodology
There were two different experiments that took place. The first of those was the Coefficient of Friction:

Equipment used : Pasco Xplorer
Any type of shoe
Surface Materials – Melamine Table, Wood, Metal, Rubber, Carpet
Scales
Calculator

•Weigh the shoe you are testing on the scales and have the weight in newtons.
•Reset the Pasco Xplorer and attach it to the shoe, either at the shoe laces or on Velcro strap.
•Place it on one of the given materials and slowly pull the shoe in a horizontal direction.
•As soon as the shoe starts to move stop and look at the given value shown on the Pasco Xplorer. This value is the Horizontal force, which is measured again in newtons (N)
• Once the horizontal force and the weight of the shoe have been calculated, the coefficient of friction (µ) can be worked out. This is done by dividing the horizontal force (Ff) by the shoe weight (N). Which is Ff/N = µ.
•Record all data down into a simple table format.

The next experiment is the angle of friction:
Equipment used : Metal Slide
Angle Indicator attached to metal slide

Various different types of shoes

Scientific Calculator
•Have all shoes lined up on the metal slide, pointing in the same direction and all levelled up.
•Slowly lift the slide upwards at the same end the shoes are on.
•Make sure to keep a close eye on the angel indicator at the bottom of the metal slide.
•Once one shoe starts to slide make a note of the angle and record it down.
•When all shoes have slid off the metal slide the experiment is over and the shoe that stayed on the longest, has he best angle of friction.
•Complete two more times to get an average score of all the data.
•To work out the coefficient of friction the formula is in tan? = µ. E,g. if the average value is 40 degrees then an40 = 8.39.
Results
Angle of Friction.
Make of Shoe Type of Shoe
Trial 1 Trial 2 Trial 3

Average Coefficient of Friction
Adidas Barricade Running 40 37 37
38 0.78
Nike air Force 1 Basketball 25 30
37 30.6 0.59
Nike Sixton Plimsole 20 20 23
21 0.38
Nike Tennis Classic Tennis 30 35 30 31.6 0.61
Asics Gel Radiance Running 30 25 40
31.6 0.61

The table shows the angle of friction results. With the highest coefficient of friction score recored was the Adidas Barricade with 0.78. The lowest score recorded was 0.38 which was achieved by the Nike Sixton. The highest angle achievd by anyone shoe was 40 and achieved by Adidas Barricade and the Asics Gel Radiance.

Appendix 1
Here is a graph to show the differences of all the different makes of shoes up against each other. As you can see the Adidas Barricade has by far the greatest coefficient of friction.

Appendix 2
Surface Shoe Weight (N) Horizontal Force (N) Coefficient of Friction
Melamine Table 2.84 2.26 0.79
Wood 2.84 1.97 0.69
Metal 2.84 2.41 0.84
Rubber 2.84 1.72 0.6
Carpet 2.84 2.38 0.83

The table shows the force needed to pull the trainer across the desired surface. There were 5 surfaces used. The surface that needed the most force for an object to be pulled across it was metal. Gaining a horizontal force of 2.41 which converted into a coefficient of friction to 0.84.

Here is a simple bar chart of the coefficient of friction scores. As seen metal and carpet were very close with scores of 0.84 and 0.83. The lowest being rubber with a score of 0.6.

Discussion
Looking at appendix 2, it shows the coefficient of friction (COF) for all different types of surfaces using the same object. With all having different values of the COF, this states the obvious, yet important point that COF depends on the nature of the surface involved. It might be argued though that the results given here are not relevant to everyday life because of the practical situation. Seeing how nobody was in the shoes to generate any real weight, then the COF could be entirely different, but it does give a clear view on what surfaces do have a high or low COF. Limiting friction is also involved in this experiment. Limiting friction is the point at which an object will eventually move if being pulled by an external force (Hay 1987). The limiting friction does depend on which two surfaces are in contact with each other and this is summarised by the law of friction: For two dry surfaces, the limiting friction is equal to the product of the force holding the surfaces together – the so called normal reaction – and a constant that depends on the nature of the surfaces (Hay 1987). This law is only correct then if the surfaces are dry. If the surfaces are then wet limiting friction is reduced. For example if a cars brakes are to become wet by rain, then it takes longer for the car to stop than if it were dry. This is why increasing the distance from the car in front is vital as, your vehicle cannot respond as well. In appendix 2 it shows that the horizontal force is greater when being pulled along on a metal surface. Recording a 2.41 Horizontal pulling force. Indicating that metal is the best surface that can hold an object onto it. But surprisingly rubber only recorded a horizontal pulling force of 1.72, which was the lowest out of all the surfaces. Persson and Volokitin (2002) state that rubber can in fact have a greater COF of more than 1. Where the results show in appendix 2 that it has only recorded a score of 0.6. With the highest being metal at 0.84.

With rubber being so low then it could be argued that the way the experiment was undertaken was wrong or not accurate. One major problem would be the horizontal pulling force. It is very difficult to pull an object only horizontally and not by pulling it in any other direction i.e. vertically. This could easily effect the results when an individual is pulling the pasco xplorer. If they only pull horizontally then it is only the horizontal force. But if you have a slight angle going up or down the horizontal force is not horizontal no more. There could be a vertical pulling force on a downward force acting on the shoe. Which will make the results inaccurate. It is very difficult to overcome this though as trying to pull an object exactly at 180 degrees so human error will always occur.

Looking at appendix 1, the angle of friction results came out with the Adidas Barricade shoe having the highest COF. While the Nike Sixton shoe had the lowest. This is mainly due to the shoes sole design and what type of tread it has. Way Li (2007) states that having tread grooves 9mm thick will result in added COF and will also aid in the shoe being able to “stick” to floor better. Which when looking at the tread of the Adidas Barricade the shoe tread was very thick compared to the Nike Sixton, which had very limited tread wear on it. Resulting in the Adidas Barricade being able to have a higher angle of friction. But Way Li (2007) also suggests that it is not the depth of the tread but more of the width. Which indicates that the Nike sixton had very little depth for tread and also very little width. Whereas the Adida Barricade had big wide treads to help it maintain the friction for longer. This is a very good point though as you could see with the Nike Air Force ones had a very deep tread but did not have the width as much as the Adidas Barricade. This designing will be down to the sport that the shoes are made for. When looking at all of the shoes in appendix 1 and what sport each shoe was specifically made for, it became clear than running shoes had a high COF. But with this could all be coincidence as the other sport involved were tennis and basketball.
The one negative factor in this test though, was how long the shoes have been used and how often. Because as said earlier the more you try and maximise friction the more energy is lost in it and results in the shoe being worn down in parts. When looking at Asics Gel Radiance, this had thicker tread wear but had been worn out around the edges. Which could be said that the more you use your shoe the less it can resist friction. As the Adidas Barricade was only purchased recently it has not been susceptible to such forces acting on it for a long period of time. Other problems did occur in the experiment e.g. when lifting the metal board up on occasions it was lifted more rapidly than other times which could of caused the shoes to jerk, which resulted in momentum taking the shoe down to the bottom. As you can see in appendix 1, the results do fluctuate often. With the Nike Air Force Ones, it begins with an angle of 25 and on the last run has a finishing angle of 37. Which is very well spread out. A way to reduce this would be to have a machine to be lifting the board up at a desired speed, very gradually with the speed staying constant.

So in conclusion the results showed that the best type of shoe to have is a shoe with thick wide tread grip on the sole. This can be mainly found with running type trainers which can be purchased in any specialist running shop. As well as having thick tread it is best to keep the shoes in good condition. As the more you use them the more the tread and grip wears down. It is stated on The Runners Guide that running trainers will last approximately last up to 300 miles. So it does take a lot for them to be fully worn out. As well as with the trainers, this study also proved that metal has the highest COF of 0.84 with the lowest being rubber of 0.6.

References
Ellis 2007 - Ellis, R. Hing, W. Reid, D. (2007). Iliotibial band friction syndrome. Manual Therapy. 12 (4).

Hammil 2003 - Hammil, J. Knutzen, K. M (2003). Biomechanical Basis of Human Movement. Philadelphia: Lippincott Williams & Wilkins.

Hay 1987 - Hay, J. G. Reid, J. G. (1987). Anatomy Mechanics and Human Motion. 2nd ed.New Jersey: Prentice Hall.
Marieb 2007 -Marieb, E. N. Hoehn, K (2007). Human Anatomy and Physiology. 7th ed. San Francisco: Pearson Education, Inc.
Persson and Volokitin (2002) -Persson, B. N. Volokitin, J. A. I.. (2002). Theory of rubber friction: Nonstationary sliding. The American Physical Society . 65 (13).
Stern 2004 - Stern, D. P. (2004). Newton's Laws of Motion. Available: www-istp.gsfc.nasa.gov. Last accessed 07 May 2010.
The Runners Guide - Hopple, J. (2008). How Long Running Shoes Last. Available: http://www.therunnersguide.com/howlongrunningshoeslast/. Last accessed 07 May 2010.
Way Li 2007 - Way Li, K. Wu, H. H. Lin, Y. C. (2007). The effect of shoe sole tread groove depth on the friction coefficient with different tread groove widths, floors and contaminants . Applied Ergonomics. 37 (6), 743-748.

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