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Related Concept Videos

Static and Kinetic Frictional Force01:05

Static and Kinetic Frictional Force

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One of the simpler characteristics of sliding friction is that it is parallel to the contact surfaces between systems, and is always in a direction that opposes the motion or attempted motion of the systems relative to each other. If two systems are in contact and moving relative to one another, then the friction between them is called kinetic friction. For example, kinetic friction slows a hockey puck sliding on ice.
However, if two systems are in contact and are stationary relative to one...
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Types of Friction Problems01:27

Types of Friction Problems

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Friction is an essential concept in physics, engineering, and everyday life. It is the force that opposes the relative motion or tendency of such motion between two surfaces in contact. One of the most common types of friction encountered in various applications is dry friction. Dry friction problems can be broadly categorized into three types, each with unique characteristics and challenges.
The first type of dry friction problem involves situations where there is no apparent impending motion....
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Kinetic Friction01:26

Kinetic Friction

1.0K
Consider a truck trying to pull a stationary car. As the truck exerts a force on the car, static friction is created at the point of contact between the two surfaces. This frictional force resists the car's movement and keeps it at rest. However, when the applied force by the truck surpasses the limiting static frictional force, an interesting phenomenon occurs. The frictional force at the interface reduces to a lower value, known as the kinetic frictional force. At this point, the car...
1.0K
Frictional Force01:07

Frictional Force

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When a body is in motion, it encounters resistance because the body interacts with its surroundings. This resistance is known as friction, a common yet complex force whose behavior is still not completely understood. Friction opposes relative motion between systems in contact, but also allows us to move. Friction arises in part due to the roughness of surfaces in contact. For one object to move along a surface, it must rise to where the peaks of the surface can skip along the bottom of the...
8.3K
Static Friction01:18

Static Friction

885
Static friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact. It plays a crucial role in our daily lives, from walking on the ground to driving a car.
For example, consider a scenario where a truck is connected to a car by a rope, ready to tow it along a road. When no external force is applied by the truck, the car remains stationary and is said to be in static equilibrium. In this case, the forces acting on the car, such as gravity and the...
885
Dry Friction01:30

Dry Friction

462
Dry friction occurs between two solid surfaces in contact as they attempt to move relative to one another. In daily life, dry friction is encountered in various forms, such as when walking on the ground, sliding an object across a table, or rubbing hands together. Despite its ubiquity, the underlying mechanisms behind dry friction are not readily visible.
To illustrate this concept, imagine a wooden crate resting on a rough, non-uniform horizontal surface. When an external force is applied to...
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Related Experiment Video

Updated: Sep 8, 2025

Preparation and Friction Force Microscopy Measurements of Immiscible, Opposing Polymer Brushes
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Preparation and Friction Force Microscopy Measurements of Immiscible, Opposing Polymer Brushes

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Active contacts control sliding friction.

Rohan Shah1, Nick Gravish1

  • 1Department of Mechanical and Aerospace Engineering, University of California, San Diego, CA 92093.

Proceedings of the National Academy of Sciences of the United States of America
|July 9, 2025
PubMed
Summary
This summary is machine-generated.

Multiple active frictional contacts can create controllable speed-dependent sliding friction, even when individual contacts are speed-independent. This finding advances understanding of locomotion and engineered surfaces.

Keywords:
active contactsfrictionlocomotionsliding

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Area of Science:

  • Physics
  • Tribology
  • Materials Science

Background:

  • Traditional Amonton-Coulomb friction law assumes speed-independent behavior.
  • Real-world systems often involve multiple frictional contacts with varying speeds.
  • Understanding multi-contact friction is crucial for locomotion and engineered surfaces.

Purpose of the Study:

  • Investigate speed-dependent sliding friction in systems with multiple active contacts.
  • Demonstrate control over friction force-speed curves using independent contact speeds.
  • Explore applications in locomotion and novel engineered surfaces.

Main Methods:

  • Experimental setup with a rotating carousel and ten speed-controlled wheels in frictional contact.
  • Systematic variation of individual contact speeds.
  • Direct measurement of sliding friction forces and system equilibrium speeds.

Main Results:

  • Multiple active contacts enable controllable speed-dependent friction, overriding individual speed-independent behavior.
  • Equilibrium system speed is determined by the median of active contact speeds.
  • Demonstrated control over the force-speed curve, effective viscosity, and friction coefficient, including near-frictionless sliding.

Conclusions:

  • Active contacts fundamentally shape dry sliding friction behavior.
  • Findings offer insights into animal and robot locomotion.
  • Opens avenues for designing engineered surfaces with tunable friction properties.