The mechanics of snow friction as revealed by micro-scale interface observations

The mechanics of snow friction are central to competitive skiing, safe winter driving and efficient polar sleds. For nearly 80 years, prevailing theory has postulated that self-lubrication accounts for low kinetic friction on snow: dry-contact sliding warms snow grains to the melting point, and further sliding produces meltwater layers that lubricate the interface. We sought to verify that self-lubrication occurs at the grain scale and to quantify the evolution of real contact area to aid modeling. We used high-resolution (15 μm) infrared thermography to observe the warming of stationary snow under a rotating polyethylene slider. Surprisingly, we did not observe melting at contacting snow grains despite low friction values. In some cases, slider shear failed inter-granular bonds and produced widespread snow movement with no persistent contacts to melt (μ < 0.03). When the snow grains did not move and persistent contacts evolved, the slider abraded rather than melted the grains at low resistance (μ < 0.05). Optical microscopy revealed that the abraded particles deposited in air pockets between grains and thereby carried heat away from the interface, a process not included in current models. Overall, our results challenge whether self-lubrication is indeed the dominant mechanism underlying low snow kinetic friction.

Evidence that abrasion can govern snow kinetic friction

The long-accepted theory to explain why snow is slippery postulates self-lubrication: frictional heat from sliding melts and thereby lubricates the contacting snow grains. We recently published micro-scale interface observations that contradicted this explanation: contacting snow grains abraded and did not melt under a polyethylene slider, despite low friction values. Here we provide additional observational and theoretical evidence that abrasion can govern snow kinetic friction. We obtained coordinated infrared, visible-light and scanning-electron micrographs that confirm that the evolving shapes observed during our tribometer tests are contacting snow grains polished by abrasion, and that the wear particles can sinter together and fill the adjacent pore spaces. Furthermore, dry-contact abrasive wear reasonably predicts the evolution of snow-slider contact area and sliding-heat-source theory confirms that contact temperatures would not reach 0°C during our tribometer tests. Importantly, published measurements of interface temperatures also indicate that melting did not occur during field tests on sleds and skis. Although prevailing theory anticipates a transition from dry to lubricated contact along a slider, we suggest that dry-contact abrasion and heat flow can prevent this transition from occurring for snow-friction scenarios of practical interest.

The Effect of Graphene Coating on Surface Roughness and Friction Properties of Polyester Fabrics

In this article, the surface roughness and friction coefficient values of graphene coated fabrics were examined. Fabrics were coated with three different graphene concentrations (5 %, 10 % and 20 %) with the knife-over-roll principle. The surface roughness of samples was measured by Accretech Surfcom 130A. Various surface roughness parameters of the coated fabrics were evaluated. Static and kinetic friction coefficients of coated fabrics were measured by Labthink Param MXD-02 friction tester using the standard wool abrasive cloth. It was observed that the coating concentration affected the frictional and roughness properties of fabrics. Experimental results showed that fabric surface roughness and friction coefficient values decreased significantly, especially at 20 % concentration. It was concluded that the coated fabrics produced could be used in applications such as anti-wear clothing.

PENGEMBANGAN PETUNJUK PRAKTIKUM GAYA GESEK DI BIDANG MIRING BERBASIS VIDEO TRACKING UNTUK MENINGKATKAN MINAT MAHASISWA

ABSTRAKPenelitian ini bertujuan menghasilkan petunjuk praktikum berbasis video tracking yang valid, praktis, dan efektif, serta mampu meningkatkan minat mahasiswa pada materi gaya gesek di bidang miring. Metode yang digunakan adalah Research and Development (R&D) dengan mengacu pada model pengembangan 4D (Define, Design, Develop dan Disseminate). Secara umum petunjuk praktikum yang dikembangkan meliputi dua topik kegiatan yakni penentuan koefisien gesek statis dengan memanipulasi kemiringan bidang serta koefisien gesek kinetis dengan menganalisis gerak benda di bidang miring. Petunjuk praktikum dalam rancangan menggunakan model praktikum/percobaan tertutup. Hasil validasi ahli yang mencakup aspek kelayakan isi, penyajian, komponen kebahasaan, dan keterlaksanaan menunjukkan petunjuk praktikum hasil pengembangan dinilai valid dengan perolehan skor rata-rata 4,03 dan layak digunakan. Hasil uji kepraktisan yang terdiri dari komponen kemenarikan, kemudahan penggunaan, dan manfaat petunjuk praktikum yang dikembangkan juga dinilai praktis dengan perolehan nilai rata-rata 3,93. Hasil uji efektifitas terhadap petunjuk praktikum menunjukkan bahwa produk pengembangan ini dinilai efektif untuk meningkatkan minat mahasiswa (kategori tinggi). Dengan demikian dapat disimpulkan bahwa produk petunjuk praktikum berbasis video tracking ini valid, praktis, efektif untuk digunakan sebagai penuntun praktikum serta mampu meningkatkan minat mahasiswa. Kata kunci: Petunjuk praktikum; video tracking; minat mahasiswa. ABSTRACTThis study is conducted to produce practical video tracking-based instructions that are valid, effective, and can increase student interest in the frictional force material on an inclined plane. The method used in this study was Research and Development (R&D) concerning the 4D development model (Define, Design, Develop and Disseminate). In general, the developed practicum instructions had covered two topics of activity, namely the determination of the coefficient of static friction by manipulating the slope of the plane and the coefficient of kinetic friction by analyzing the motion of objects on an inclined plane. Practical instructions in the design adopted a closed practicum/experimental model. The results of expert validation which include aspects of the feasibility of content, presentation, linguistic components, and implementation indicated that the practicum instructions developed were estimated valid with an average score of 4.03 and were suitable for use. On the other hand, the results of the practicality test consisting of components of attractiveness, ease of use, and the benefits of the developed practicum instructions were also considered practical with an average score of 3.93. Moreover, the results of the effectiveness test on the practicum instructions showed that this development product was considered effective for increasing student interest (high category). Thus, it can be concluded that this video tracking-based practicum guide product is valid, practical, effective to be used as a practicum guide, and is able to increase student interest. Keywords: Practical Instructions; video tracking; student interest.

Using Accelerometer Smartphone Sensor and Phyphyox for Friction Experiment in High School

Experiments related to friction in schools are mostly done by direct observation, and it becomes more difficult to determine the value of the coefficients of static and kinetic friction due to the low level of accuracy. This study aims to develop a friction coefficient practicum tool using the accelerometer sensor on a smartphone with the Phyphox application. The research was conducted using the 4D Thiagarajan method (Define, Design, Develop, Disseminate). This tool demonstrates experimental friction on an inclined plane with a variety of surface materials (aluminium, wood, glass, acrylic). The results recorded by Phyphox are spreadsheets of object acceleration data per unit time in x, y, and z coordinate graphs. The sliding plane frame is designed using an Arduino microcontroller which can make it easier for students to automatically form inclined plane angles. This tool has been tested by experts with an average score are 85.25% and has been tested on physics teachers with an average score of 82.5% in the good category. This friction coefficient practicum tool is feasible and can help students understand the concept of friction

A method for determining the high-temperature coefficient of friction

A method for estimating the coefficient of friction at high temperatures up to 220°C in the thermal chamber of a bursting machine has been developed. It is shown that the coefficient of kinetic friction with a change in temperature from 25°C to 220°C varies from 0.04 to 0.1. In the developed method, the coefficient of static friction and the coefficient of kinetic friction are determined. The coefficient of static friction at a temperature of 25°C to 220°C varies from 0.06 to 0.13.