ES 370 Friction Lab Biomechanics I Friction opposes motion and yet without friction movement as we know if would be impossible The forces between two objects in contact are resolved into the normal reaction force and friction force Recall the normal reaction force in the force at a right angle perpendicular to the surfaces in contact while friction is force acting in parallel to the surfaces When the two surfaces are dry the force of friction F is equal to the product of the coefficient of friction and the normal reaction force FN or F FN Interestingly friction force can be a resistance Newton s 1st law and a propulsive force Newton s 2nd law In this lab we will be examining friction force from both perspectives Typical coefficients of friction in human movement or sport vary widely as they depend on the texture and nature of the two surfaces Athletic shoes have coefficients of static friction ranging from 0 4 to over 2 0 depending on the shoe and sport surface Kinetic coefficient of friction is lower than static coefficient of friction by about 25 Maximum limiting friction is defined as the maximum friction force Fm that can be developed between two materials in contact before sliding occurs e g overcoming the static coefficient of friction The kinetic or sliding coefficient of friction measured between ice and a speed skating blade is around 0 005 by comparison Consider the design of the outsole for footwear used in softball or baseball football curling basketball mountain climbing hockey and wrestling and their respective athletic surfaces Next consider whether increasing or decreasing friction force would benefit performance or injury risk These considerations drive the sport apparel shoe and athletic surface research and development industries as they manipulate friction force specifically coefficient of friction to reduce injury and increase performance Purpose 1 Explore the critical role of friction in human movement using the Tug of War example 2 Determine the static coefficient of friction for various shoe surface interfaces 1 In small sided teams complete one round of tug of war 2 The same teams will complete a second round of tug of war However in round 2 the winning team will remove their shoes and complete this round wearing only their socks neither bare feet nor shoes allowed 3 The basement of Kimball is a good location for this task Procedures Task 1 Tug of War Biomechanics I ES 370 Task 2 Shoe surface interface evaluation In this exercise we will examine factors that influence static coefficients of friction on a variety of shoe surface combinations We will be using the following playing surface types artificial turf cement hardwood carpet and linoleum on several types of athletic shoes 1 In pairs record the weight of the shoe and the foot i e the small sandbag inside the shoe Shoe Basketball Hiking Grass cleat Turf cleat Shoe foot mass N 8 829 9 81 7 848 5 886 Note Convert kg to Newton s N by multiplying kg x gravity 9 81 m s2 2 Place one shoe onto a playing surface 3 Slowly lift the end of the playing surface board and note the instant that the shoe begins to slide on the playing surface and use the goniometer or inclinometer most smartphones have this feature to record the angle of the tilted board relative to the tabletop 4 Carefully repeat the process and then calculate the average of the two measured angles 5 Complete this process for all shoe surface interface permutations 6 With the weight of the shoe and the angle at which the shoe moved calculate the static coefficient of friction for each shoe surface permutation Be sure to include this table 1 in your lab report Shoe type Basketball Hiking Grass cleat Turf cleat Shoe Surface Angles Artificial Turf Cement Carpet Linoleum Hardwood 50 45 41 57 42 43 43 45 45 42 43 58 30 36 28 32 38 42 37 40 Biomechanics I ES 370 Lab Report Questions 18 points 1 During the tug of war explain whether friction was acting as a propulsive force resistance force or both What about during task 2 2 pts Propulsive force In task 2 it was a resistance force 2 Why did the losing team from round 1 win round 2 1 pt 3 Which athletic shoe surface combination had the highest coefficient of static friction 1pt Turf cleat and carpet 4 Which athletic shoe surface combination had the lowest friction force 1 pt Hiking shoes and linoleum 5 How does one avoid slipping when walking on a surface with a low coefficient of friction Hint draw a diagram showing friction force and the vertical and horizontal components of one s foot contact vector 2 pts shoes with good traction are essential Look for shoes with rubber soles that have a pattern designed for grip Shoes with worn out soles should be replaced as they may not provide enough traction a What other strategies can one do to increase friction between the feet and an icy floor 2 pts Sprinkling sand salt or even cat litter on the icy surface can provide more grip These substances create a rougher surface increasing friction 6 Name two sports where increasing friction would be beneficial Name two sports where athletes purposely decrease friction force 2 pts INCREASE FRICTION Basketball Increasing friction on basketball shoes is beneficial for quick cuts stops and changes in direction on the court Rock Climbing Climbing shoes are designed to increase friction on rock surfaces allowing climbers to grip small holds and ledges DEC FRICTION Swimming Swimmers often wear specialized suits that reduce friction with the water allowing for smoother and faster movement through the pool ES 370 Biomechanics I Speed Skating Speed skaters use specialized suits and equipment to minimize friction with the ice allowing them to glide faster across the surface 7 Epidemiological studies have shown that playing on lower friction surfaces such as clay versus hardcourt tennis courts lower injury risk Nigg et al 1989 Discuss whether you believe this injury risk tradeoff is worthwhile in tennis hint consider game speed and athlete performance in addition to injury risk 3 pts Compared to hard courts clay courts are softer and offer greater cushioning which can lessen the strain on joints particularly for athletes who already have joint problems Clay courts offer a slower paced game that allows players to react and recover more slowly which may lessen the chance of acute injuries like sprained ankles or strained muscles Compared to hard surfaces falls and slides on clay courts frequently cause fewer skin injuries and