Name: dry friction in crate, ladder, and frame problems
Uploaded: 2025-05-22T09:06:04.000Z
Duration: 1 min 27 s
Channel: JoVE

Les problèmes de friction à sec peuvent être classés en trois types.
Le premier type est le mouvement d’absence d’imminence apparente. Considérons une force appliquée à une caisse.
Il y a trois inconnues, qui peuvent être évaluées à partir des trois équations d’équilibre. Ainsi, la force de frottement développée peut être déterminée.
Le deuxième type est le mouvement imminent à tous les points de contact, par exemple, une échelle placée contre un mur lisse.
Les quatre inconnues peuvent être déterminées à partir des trois équations d’équilibre et d’une équation de frottement statique qui s’appliquent au point de contact.
Ensuite, le plus petit angle auquel l’échelle peut être placée sans glisser est évalué.
Le troisième type est le mouvement imminent à certains points de contact. Considérons un cadre à deux membres soumis à une force horizontale inconnue.
Il y a sept inconnues, six équations d’équilibre et l’une des deux équations de frottement statique possibles. En les résolvant, la force horizontale nécessaire pour provoquer le mouvement peut être déterminée.
Lorsque nous augmentons la force, deux possibilités se présentent, glisser à l’extrémité gauche et non à l’extrémité droite, ou vice versa.

<ol>
	<li>Hibbeler, R.C. (2016). Mécanique du génie : Statique. Quatorzième édition, New Jersey : Pearson. P. 406 à 412.</li>
	<li>Meriam, J.L., Kraige, L.G. et Bolten, J.N. (2016). Mécanique de l’ingénierie : statique et dynamique. Huitième édition, Singapour : John Wiley & Sons. p. 337.</li>
</ol>

types de problèmes de friction

Types of Friction Problems

La friction est un concept essentiel en physique, en ingénierie et dans la vie quotidienne. C'est la force qui s'oppose au mouvement relatif ou à la tendance à ce mouvement entre deux surfaces en contact. L'un des types de friction les plus courants rencontrés dans diverses applications est la friction sèche. Les problèmes de friction sèche peuvent être largement classés en trois types, chacun avec des caractéristiques et des défis uniques. Le premier type de problème de friction sèche concerne les situations où il n'y a pas de mouvement imminent apparent. Un exemple typique de cela est lorsqu'une force est appliquée à une caisse mais reste stationnaire. Les forces de friction développées entre le sol et la caisse s'opposent à la force appliquée et maintiennent la caisse immobile. Pour résoudre ce type de problème, il faut prendre en compte les trois inconnues impliquées : la force normale, la force de friction et la force appliquée. Ces inconnues peuvent être évaluées en utilisant les trois équations d'équilibre qui régissent le système. Une fois ces inconnues déterminées, la force de friction développée peut être calculée, ce qui permet de mieux comprendre les facteurs affectant la stabilité de la caisse. Le deuxième type de problème de friction sèche concerne les situations où un mouvement imminent se produit à tous les points de contact. Un exemple de ce problème est une échelle placée contre un mur lisse. Dans ce scénario, l'échelle est sur le point de glisser et il faut déterminer le plus petit angle auquel l'échelle peut être placée sans glisser. Pour résoudre ce problème, il faut d'abord identifier les quatre inconnues présentes : les forces normales aux deux points de contact, la force de friction à la base de l'échelle et l'angle. Ces inconnues peuvent être déterminées en utilisant les trois équations d'équilibre et une équation de friction statique qui s'applique aux points de contact. Une fois ces valeurs calculées, le plus petit angle auquel l'échelle peut être placée sans glisser peut être évalué. Le troisième type de problème de friction sèche concerne les situations où un mouvement imminent se produit à certains points de contact, mais pas à tous. Un exemple de ce type de problème est une structure à deux membres soumis à une force horizontale inconnue. Dans ce cas, la structure peut glisser à une extrémité tout en restant immobile à l'autre. Pour résoudre ce problème, il faut identifier les sept inconnues impliquées : les forces normales aux deux points de contact, les forces de friction aux deux extrémités, la force horizontale appliquée et les forces de réaction aux supports. Six équations d'équilibre et l'une des deux équations de friction statique possibles peuvent être utilisées pour déterminer ces inconnues. Résoudre ces équations peut déterminer la force horizontale nécessaire pour provoquer un mouvement dans la structure. Lorsque la force appliquée à la structure augmente, deux possibilités peuvent se produire : glissement à l'extrémité gauche tandis que l'extrémité droite reste immobile, ou vice versa. En analysant ces possibilités, on peut mieux comprendre les facteurs influençant le mouvement de la structure et les conditions dans lesquelles elle glissera.

Types de problèmes de friction

La friction est un concept essentiel en physique, en ingénierie et dans la vie quotidienne. C’est la force qui s’oppose au mouvement relatif ou à la tendance d’un tel mouvement...

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Friction

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dry friction in crate, ladder, and frame problems

Dry friction problems often focus on crates, ladders, and frames that are close to slipping. Dry friction is the force that resists relative motion, or the tendency for motion, between two surfaces in contact. In many physics and engineering problems, this force helps keep an object still or defines the point where motion is about to begin.

One common case has no clear impending motion. A force may be applied to a crate, but the crate stays at rest. The friction force between the floor and the crate opposes the applied force and keeps the crate stationary. To solve this kind of problem, the normal force, the friction force, and the applied force are the three unknowns. They can be found using the three equilibrium equations for the system.

A second type of problem reaches impending motion at every contact point. A ladder against a smooth wall is a good example. The ladder is just about to slip, and the goal is to find the smallest angle that still keeps it from sliding. This setup has four unknowns: the normal forces at both contact points, the friction force at the base, and the angle. The three equilibrium equations and one static friction equation are used to solve for these values.

A third type involves impending motion at some contact points, but not at others. A two-member frame with an unknown horizontal force is one example. The frame may slip at one end while the other end remains stationary. This case has seven unknowns, including the normal forces, the friction forces at both ends, the applied horizontal force, and the reaction forces at the supports. Six equilibrium equations and one of the two possible static friction equations are used to determine the force needed to start movement. As the applied force increases, the frame may slip on the left while the right side stays fixed, or the reverse may happen.

Dry Friction in Crate, Ladder, and Frame Problems

dry-friction-in-crate-ladder-and-frame-problems

dry friction

Dry Friction in Crates and Everyday Motion

Dry friction appears when two solid surfaces touch and try to move past each other. Students can see it when they walk, slide a box across a table, or rub their hands together. Even though it is common, the forces behind it are not easy to see. A simple crate on a rough horizontal floor helps explain the idea. When an external force pushes the crate sideways, two main forces act on it. The normal force acts perpendicular to the surface and helps balance the crate’s weight, while the frictional...

static friction

Static Friction at the Point of Slip

Static friction is the force that keeps two surfaces from starting to slide past each other. It acts when there is a tendency for motion, but not yet actual slipping. This force helps explain everyday actions like walking on the ground and driving a car. A towing example shows how static friction changes as force is applied. A truck pulls a car with a rope, and the car stays still at first. The car is in static equilibrium, which means the forces on it are balanced. Gravity and the normal...

kinetic friction

Sliding Friction Between Moving Surfaces

Kinetic friction is the sliding friction that acts when two surfaces move against each other. It opposes relative motion and acts parallel to the contact surface, in the opposite direction of travel. In the truck and car example, the car first stays still because static friction holds it in place. When the truck’s force becomes greater than the limiting static frictional force, the friction at the contact surface drops to a lower value. The car then begins to slide, and its speed increases. At...

characteristics of dry friction

Static and Kinetic Dry Friction

Dry friction is the resistance between two solid surfaces that move against each other without lubrication or any fluid. A common example is a gardener pushing a wheelbarrow across the ground. The wheel and the ground create dry friction that slows the motion. Before the wheelbarrow starts to move, static friction acts at the contact surface. Static friction is the force that opposes motion before slipping begins, and it helps keep the wheelbarrow stable. It is present only when there is no...

types of friction problems

Dry friction problems often focus on crates, ladders, and frames that are close to slipping. Dry friction is the force that resists relative motion, or the tendency for motion, between two surfaces in contact. In many physics and engineering problems, this force helps keep an object still or defines the point where motion is about to begin. One common case has no clear impending motion. A force may be applied to a crate, but the crate stays at rest. The friction force between the floor and the...

friction: problem solving

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upward impending motion

Screw Jack Force and Friction Analysis

A square-threaded screw jack lifts heavy loads by turning handle force into a twisting moment. Its upward motion depends on overcoming static friction between the screw threads and the jack. To study this motion, the thread can be treated as if it were a block on the groove of the jack after the thread is completely unraveled. A free-body diagram helps show the forces acting on this simplified system. The force at the handle creates a horizontal force that pushes the movable thread up the...

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Screw Jack Self-Locking Behavior

A square-threaded screw jack can hold a heavy load because of its self-locking behavior. This feature makes the device more reliable when it is used to lift large weights or apply a strong force. If the screw keeps its position after the turning moment is removed, the jack is self-locking. Self-locking happens because friction changes the way the forces act on the thread. The friction force reverses direction, and the reaction force moves to the other side of the thread’s normal. In this case,...

screw: problem solving

Screw Torque and Self-Locking

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frictional forces on flat belts

Flat Belt Tension and Friction

Flat belt tension and friction control how power moves from one pulley to another. Flat belts are used in many industrial systems to transmit power between pulleys. As the belt wraps around the pulleys, the tension changes because of friction between the belt and the pulley surface. When a pulley turns counterclockwise, the belt side moving away from the pulley has the higher tension, called T2. The side moving toward the pulley has the lower tension, called T1. To estimate these tensions, the...

flat belts: problem solving

Flat Belt Forces and Pulley Moment

Flat belts transmit power from one pulley to another, and the forces on the belt help determine the moment on a pulley. In this example, a belt wraps around pulleys A and B, with radii of 30 cm and 10 cm. The belt meets the pulleys at a 20 degree angle from the horizontal, which sets up the force balance in the system. As pulley B rotates clockwise, it drives pulley A. That motion creates tension T2 on one side of the belt and tension T1 on the other side. The belt-to-surface contact angle, β,...

pivot bearings

Axial Load Support in Pivot Bearings

Pivot bearings support axial loads on a rotating shaft. A bearing helps two parts move relative to each other while reducing friction and wear. In this design, the pivot is placed at the end of the shaft to carry the axial thrust. The pivot surface can be flat, conical, or truncated, depending on the application. In a flat pivot bearing, the axial load is spread over the bearing area. The pressure is uniform and can be found by dividing the load by the area. To study the forces more closely,...

collar bearings

Friction Torque in Collar Bearings

Collar bearings support axial loads on rotating shafts, and their friction helps determine the torque needed to start motion. They are used in many machines and may have a single collar or multiple collars, depending on the application and load needs. For a single collar bearing under an axial load, the contact area is the ring-shaped region between the external radius and the internal radius of the collar. If the bearing is assumed to support the load evenly, the uniform normal pressure is...

journal bearings

Friction Moment in Journal Bearings

Journal bearings support rotating shafts and axles while helping keep them stable. They are used in machines such as engines, turbines, and pumps. Their job is to reduce friction, wear, and vibration. A journal bearing works by keeping a thin film of lubricant between the bearing surface and the rotating shaft. This film limits direct contact and lowers frictional force. The video uses a rope winch with dry or only partly lubricated journal bearings to show the idea. As the spool turns...

bearings: problem solving

Double-Collar Bearing Torque Calculations

Double-collar bearing torque calculations show how friction and axial force affect bearing performance. Engineers use these calculations to test whether the bearing can handle the forces and moments it experiences during operation. This helps them choose and design bearings for specific applications and setups. For a double-collar bearing with set dimensions and an axial force applied, the maximum frictional moment can be found with a standard expression. The equation is rearranged, and the...

rolling resistance

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rolling resistance: problem solving

Lawn Roller Force Calculation

Rolling resistance is the force that makes a rolling object harder to move across a surface. It is also called rolling friction. It comes from deformation in the object and the surface that touch each other. Internal friction, hysteresis, and other energy losses in the materials also play a role. Because of this force, extra energy is needed to keep a wheel, tire, or ball moving. The video applies this idea to a lawn roller. The roller has a mass of 100 kg, a radius of 25 cm, and a coefficient...

key terms and concepts

Explore dry friction through crate, ladder, and frame problems. Learn how equilibrium and static friction equations help predict when slipping begins.

Dry Friction in Crate, Ladder, and Frame Problems | JoVE K12

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

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Dry Friction in Crate, Ladder, and Frame Problems

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