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Concept: Normal force


SignificanceThe goal of this study was to use the Surface Forces Apparatus to examine the effects of slip-stick friction on cartilage surface morphology under different loading and sliding conditions. Different load and speed regimes were represented using friction maps that separated regimes of smooth and stick-slip sliding. The finding of this work is that damage generally occurs within the stick-slip regimes and is not directly related to the friction coefficient. Prolonged exposure of cartilage surfaces to stick-slip sliding resulted in a significant increase of surface roughness, indicative of severe morphological changes (damage) of the cartilage surfaces.

Concepts: Force, Classical mechanics, Friction, Road, Tribology, Roughness, Normal force, Stick-slip phenomenon


Friction, wear and the associated energy dissipation are major challenges in all systems containing moving parts. Examples range from nanoelectromechanical systems over hip prosthesis to off-shore wind turbines. Bionic approaches have proven to be very successful in many engineering problems, while investigating the potential of a bio-inspired approach in creating morphological surface textures is a relatively new field of research. Here, we developed laser-created textures inspired by the scales found on the skin of snakes and certain lizards. We show that this bio-inspired surface morphology reduced dry sliding friction forces by more than 40%. In lubricated contacts the same morphology increased friction by a factor of three. Two different kinds of morphologies, one with completely overlapping scales and one with the scales arranged in individual rows, were chosen. In lubricated as well as unlubricated contacts, the surface texture with the scales in rows showed lower friction forces than the completely overlapping ones. We anticipate that these results could have significant impact in all dry sliding contacts, ranging from nanoelectromechanical and micro-positioning systems up to large-scale tribological contacts which cannot be lubricated, e.g. because they are employed in a vacuum environment.

Concepts: Mass, Force, Classical mechanics, Friction, Tribology, Texture, Normal force, Lubricant


Powder flow of mixtures is complex and not properly understood. The selection of drug-excipient blends with inadequate powder flow can lead to quality issues of the final dosage form. Therefore, this work aims at a better understanding of how changes in powder flow of binary blends can lead to weight variability in pharmaceutical capsule filling. We used image-analysis-based powder avalanching and shear cell testing to study blends of paracetamol and microcrystalline cellulose. A pilot-scale machine with dosator principle was employed for encapsulation. As a result, the powder flow properties improved generally with rising amounts of microcrystalline cellulose. However, a negative correlation was observed between avalanche angle and angle of internal friction. Results were discussed and percolation theory was considered to explain abrupt changes in the observed flow properties. This was particularly helpful for analysis of the capsule-filling data, since capsule weight variability displayed a threshold behavior as a function of the mixture fraction. The capsule weight variability correlated with the angle of internal friction as well as with the angle and the energy of avalanches. Based on the results we proposed a strategy of how to design minimal weight variability into powder-filled capsules.

Concepts: Understanding, Mass, Force, Knowledge, Explanation, Normal force, Avalanche, Avalanches


The primary role of the shoulder joint in tennis forehand drive is at the expense of the loadings undergone by this joint. Nevertheless, few studies investigated glenohumeral (GH) contact forces during forehand drives. The aim of this study was to investigate GH compressive and shearing forces during the flat and topspin forehand drives in advanced tennis players. 3D kinematics of flat and topspin forehand drives of 11 advanced tennis players were recorded. The Delft Shoulder and Elbow musculoskeletal model was implemented to assess the magnitude and orientation of GH contact forces during the forehand drives. The results showed no differences in magnitude and orientation of GH contact forces between the flat and topspin forehand drives. The estimated maximal GH contact force during the forward swing phase was 3573 ± 1383 N, which was on average 1.25 times greater than during the follow-through phase, and 5.8 times greater than during the backswing phase. Regardless the phase of the forehand drive, GH contact forces pointed towards the anterior-superior part of the glenoid therefore standing for shearing forces. Knowledge of GH contact forces during real sport tasks performed at high velocity may improve the understanding of various sport-specific adaptations and causative factors for shoulder problems.

Concepts: Force, Classical mechanics, Shoulder, Euclidean vector, Deltoid muscle, Normal force, Glenohumeral joint, Contact force


Any macroscopic deformation of a filamentous bundle is necessarily accompanied by local sliding and/or stretching of the constituent filaments. Yet the nature of the sliding friction between two aligned filaments interacting through multiple contacts remains largely unexplored. Here, by directly measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly large, scale logarithmically with sliding velocity as in solid-like friction, and exhibit complex dependence on the filaments' overlap length. We also show that a reduction of the frictional force by orders of magnitude, associated with a transition from solid-like friction to Stokes’s drag, can be induced by coating F-actin with polymeric brushes. Furthermore, we observe similar transitions in filamentous microtubules and bacterial flagella. Our findings demonstrate how altering a filament’s elasticity, structure and interactions can be used to engineer interfilament friction and thus tune the properties of fibrous composite materials.

Concepts: Eukaryote, Mass, Force, Cytoskeleton, Classical mechanics, Friction, Filament, Normal force


Amontons' law defines the friction coefficient as the ratio between friction force and normal force, and assumes that both these forces depend linearly on the real contact area between the two sliding surfaces. However, experimental testing of frictional contact models has proven difficult, because few in situ experiments are able to resolve this real contact area. Here, we present a contact detection method with molecular-level sensitivity. We find that while the friction force is proportional to the real contact area, the real contact area does not increase linearly with normal force. Contact simulations show that this is due to both elastic interactions between asperities on the surface and contact plasticity of the asperities. We reproduce the contact area and fine details of the measured contact geometry by including plastic hardening into the simulations. These new insights will pave the way for a quantitative microscopic understanding of contact mechanics and tribology.

Concepts: Mass, Force, Classical mechanics, Introductory physics, Solid mechanics, Friction, Tribology, Normal force


Understanding how body-borne mass influences knee loads during running and how to modulate these knee loads may assist efforts to reduce the high rate of knee injuries in military populations. We tested a) the extent a 15-kg body-borne load affects peak and cumulative patellofemoral (PFJ) and tibiofemoral (TFJ) contact forces during running and b) if a 7.5% increase in running cadence modulates these contact forces. Compared with unloaded running, the body-borne load increased peak PFJ contact force (+0.2 body weights; p=0.001) and PFJ impulse (+32 body weights per km; p<0.001). Additionally, greater peak total TFJ contact force (+0.5 body weights; p<0.001) and greater peak medial TFJ contact force (+0.4 body weights; p=0.002) resulted with the added load. Similarly, 85 additional body weights of total TFJ impulse per km (p<0.001) and 65 additional body weights of medial TFJ impulse per km (p<0.001) were noted with the added load. The higher cadence condition reduced peak PFJ force (-0.5 body weights, p<0.001) and PFJ impulse per km (-15 body weights per km, p<0.016). Reduced peak total and peak medial TFJ contact forces (-0.8 body weights, p<0.001; -0.5 body weights, p<0.001, respectively) were also found with higher cadence, while reduced total TFJ and medial TFJ impulse per km (-18.5 body weights per km, p<0.001; -12.2 body weights per km, p<0.001, respectively) were observed. Thus, running with increased cadence eliminated increased knee loads per step but only partially reduced the greater cumulative knee loads per km that resulted with an added 15-kg body-borne load.

Concepts: Mass, Force, Space, Classical mechanics, The Higher, Weight, Normal force, Contact force


In many sliding systems consisting of solid object on a solid substrate under dry condition, the friction force does not depend on the apparent contact area and is proportional to the loading force. This behaviour is called Amontons' law and indicates that the friction coefficient, or the ratio of the friction force to the loading force, is constant. Here, however, using numerical and analytical methods, we show that Amontons' law breaks down systematically under certain conditions for an elastic object experiencing a friction force that locally obeys Amontons' law. The macroscopic static friction coefficient, which corresponds to the onset of bulk sliding of the object, decreases as pressure or system length increases. This decrease results from precursor slips before the onset of bulk sliding, and is consistent with the results of certain previous experiments. The mechanisms for these behaviours are clarified. These results will provide new insight into controlling friction.

Concepts: Mass, Ratio, Force, Classical mechanics, Friction, Normal force, Contact area, Guillaume Amontons


The purpose of this study was to examine the effects of step length and foot strike pattern along with their interaction on tibiofemoral joint (TFJ) and medial compartment TFJ kinetics during running. Nineteen participants ran with a rear foot strike pattern at their preferred speed using a short (-10%), preferred, and long (+10%) step length. These step length conditions were then repeated using a forefoot strike pattern. Regardless of foot strike pattern, a 10% shorter step length resulted in decreased peak contact force, force impulse per step, force impulse per kilometre, and average loading rate at the TFJ and medial compartment, while a 10% increased step length had the opposite effects (all P < 0.05). A forefoot strike pattern significantly lowered TFJ and medial compartment TFJ average loading rates compared with a rear foot strike pattern (both <0.05) but did not change TFJ or medial compartment peak force, force impulse per step, or force impulse per km. The combination of a shorter step length and forefoot strike pattern produced the greatest reduction in peak medial compartment contact force (P < 0.05). Knowledge of these running modification effects may be relevant to the management or prevention of TFJ injury or pathology among runners.

Concepts: Orders of magnitude, Kilometre, Force, Height, Length, Normal force, Astronomical unit, Light-year


Understanding the response of granular matter to intrusion of solid objects is key to modelling many aspects of behaviour of granular matter, including plastic flow. Here we report a general model for such a quasistatic process. Using a range of experiments, we first show that the relation between the penetration depth and the force resisting it, transiently nonlinear and then linear, is scalable to a universal form. We show that the gradient of the steady-state part, Kϕ, depends only on the medium’s internal friction angle, ϕ, and that it is nonlinear in μ = tan ϕ, in contrast to an existing conjecture. We further show that the intrusion of any convex solid shape satisfies a modified Archimedes' law and use this to: relate the zero-depth intercept of the linear part to Kϕand the intruder’s cross-section; explain the curve’s nonlinear part in terms of the stagnant zone’s development.

Concepts: Atom, Mass, Force, Solid, Classical mechanics, Friction, Normal force, Guillaume Amontons