Name: calculating force moments in problem solving
Uploaded: 2023-09-22T11:49:20.000Z
Duration: 1 min 29 s
Channel: JoVE

Prenons l’exemple d’un homme qui arrache un clou d’un mur en bois à l’aide d’un pied-de-biche. Le pied-de-biche entre en contact avec le mur au point A. L’homme applique une force verticaleF1 sur la prise du pied-de-biche, alors qu’il faut une forceF2 sur la griffe pour retirer le clou. Déterminez la force minimale dont l’homme a besoin pour retirer le clou.
Au point A, le moment dû à la force F1 peut être exprimé comme la somme du produit de l’intensité de la force et des distances perpendiculaires. En substituant les valeurs, on peut déterminer le moment en A dû à F1.
De même, en résolvant la force F2 à la griffe en ses composantes, le moment dû à F2 au point A peut être déterminé à l’aide de la composante sinusoïdale de F2.
Étant donné que le moment de force,F1 est plus petit que F2, la force F1 est insuffisante pour enlever le clou.
En comparant à la fois les équations du moment de force et en substituant la valeur, la force minimale nécessaire pour retirer le clou peut être déterminée.

<ol>
	<li>Russell C. Hibbeler. (2016). Mécanique de l’ingénierie : statique et dynamique. Pearson. N° de page - 121-124.</li>
</ol>

moment d'une force : résolution de problème

Comprendre la formulation scalaire du moment d'une force et l'appliquer correctement dans la résolution de problèmes est essentiel pour la conception et l'analyse de systèmes mécaniques. Voici les étapes pour résoudre un problème avec le moment d'une force : <ol> <li>Dessinez un diagramme de corps libre (FBD) du système. Le FBD est une étape de résolution de problème de base. C'est une représentation diagrammatique de toutes les forces agissant sur le système. Chaque force agissante sur le système doit être identifiée et incluse dans le FBD.</li> <li>Identifiez l'axe ou le point autour duquel le moment est calculé. Dans certains cas, l'axe ou le point autour duquel le moment net doit être calculé est fourni. Cependant, dans d'autres cas, le problème nécessite de choisir un axe ou un point approprié.</li> <li>Calculez la distance perpendiculaire entre la force et l'axe ou le point. La distance perpendiculaire est la distance la plus courte entre la force et l'axe ou le point choisi.</li> <li>Calculez le moment de chaque force. À cette étape, les magnitudes des forces individuelles et leurs distances par rapport à l'axe ou au point choisi sont multipliées pour calculer leurs moments respectifs. Ici, le moment d'une force est une quantité vectorielle, et sa direction est perpendiculaire au plan de la force et à l'axe ou au point choisi.</li> <li>Additionnez les moments. Une fois que les moments de toutes les forces ont été calculés, ils doivent être additionnés de manière algébrique pour obtenir le moment net agissant sur le système.</li> <li>Déterminez l'effet du moment net. Si le moment net est nul, il n'y a pas de mouvement de rotation. Si le moment net est négatif, l'objet tournera dans le sens des aiguilles d'une montre, et si le moment net est positif, l'objet tournera dans le sens inverse des aiguilles d'une montre.</li> <li>Résolvez le problème. La dernière étape de la résolution de problème consiste à utiliser le moment net calculé pour résoudre le problème donné. Par exemple, si l'accélération angulaire d'un objet doit être calculée, le moment net agissant sur l'objet est divisé par le moment d'inertie de l'objet.</li> </ol>

Moment d'une force : Résolution de problème

Comprendre la formulation scalaire du moment d’une force et l’appliquer correctement à travers la résolution de problèmes est crucial pour concevoir et analyser ...

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Force System Resultants

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Moment of a Force: Problem Solving

calculating force moments in problem solving

Calculating force moments in problem solving helps students analyze rotation in mechanical systems. The scalar form of the moment of a force is used to work through these problems step by step.

Start by drawing a free-body diagram, or FBD. An FBD is a sketch that shows every force acting on the system. Each force must be identified and included.

Next, identify the axis or point about which the moment is calculated. In some problems, that axis or point is given. In other cases, you must choose a suitable one. Then find the perpendicular distance, which is the shortest distance between the force and the axis or point.

After that, calculate the moment of each force. This is done by multiplying the force magnitude by its perpendicular distance. The moment is a vector quantity, and its direction is perpendicular to the plane of the force and the chosen axis or point.

Add the moments algebraically to find the net moment. Then interpret the result. If the net moment is zero, there is no rotational motion. If it is negative, the object rotates clockwise. If it is positive, the object rotates counterclockwise.

Use the net moment to solve the problem. For example, if angular acceleration is needed, divide the net moment by the moment of inertia of the object.

Calculating Force Moments in Problem Solving

calculating-force-moments-in-problem-solving

moment of a force: scalar formulation

Torque on a Flywheel: Direction and Size

Torque, or the moment of a force, describes how a force can make an object turn about an axis. It is a vector quantity, so it has both size and direction. This idea is important in physics, mechanics, and engineering. A simple flywheel example shows how torque works. The flywheel rotates about point O when a force is applied to it. The moment arm is the perpendicular distance from point O to the line of action of the force. The moment, in newton-meter (N-m), is found by multiplying the force...

moment of a force: problem solving

Calculating force moments in problem solving helps students analyze rotation in mechanical systems. The scalar form of the moment of a force is used to work through these problems step by step. Start by drawing a free-body diagram, or FBD. An FBD is a sketch that shows every force acting on the system. Each force must be identified and included. Next, identify the axis or point about which the moment is calculated. In some problems, that axis or point is given. In other cases, you must choose...

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Net Moment Sign Convention in 2D Forces

Net moment in two-dimensional force systems shows how multiple forces can cause rotation about a fixed point. The scalar formulation helps analyze equilibrium in structures that are acted on by several forces. It focuses on the x-y plane and uses a simple sign rule to describe the direction of rotation. To find the resultant moment, the moments from all forces in the system are considered about a chosen point A. Each moment is assigned a sign based on its sense of rotation. Positive moments...

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Calculating Moment with Cartesian Vectors

Calculating moment with Cartesian vectors shows how position and force vectors work together. The moment of force is the cross product of these vectors, so it is a vector quantity. In Cartesian form, unit vectors help write the vectors in component form. The cross product can be arranged in determinant form. That determinant is then expanded to find the Cartesian vector form of the moment of force. This method gives a clear way to work through the calculation step by step. A revolving door...

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Resultant moment describes the total turning effect of multiple forces acting on an object. It shows how several forces can combine to make a body rotate about a point. In physics, moment is the tendency of a force to cause rotation. The resultant moment of a force system is a vector quantity. That means it can be added and subtracted like other vectors. For a system with two forces, F1 and F2, acting on a pole at points A and B, the resultant moment is written as MR = (rA x F1) + (rB x F2).

principle of moments

Resolving Forces in Moment Calculations

The principle of moments, also called Varignon's theorem, explains how a force creates rotation about a point. It is a key idea in physics and engineering. It also helps describe when a rigid body stays in equilibrium under external forces. The moment of a force is found by multiplying the force by the perpendicular distance from the point of application to the point being studied. A larger force or a larger distance gives a larger moment. This makes the idea useful for checking how strongly a...

principle of moments: problem solving

Torque About an Axis in 3D Systems

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moment of a force about an axis: scalar

Bicycle Torque and Moment About an Axis

The moment of a force about an axis describes how a force can make an object turn around a fixed line. In scalar analysis, the moment is found from the perpendicular distance between the axis of rotation and the line of action of the force. That distance is called the moment arm. A bicycle gives a clear example of this idea. When the cyclist pushes on the pedal, the chain applies a force to the wheel’s axle and makes it rotate. The axis of rotation passes through the axle, and the moment arm...

moment of a force about an axis: vector

Torque on a Bicycle Wheel: Vector Method

Torque, or the moment of a force, explains how a force can make an object rotate about an axis. A bicycle is a clear example. When a cyclist presses on the pedal, the crankshaft turns and helps drive the wheel. To analyze this motion, a coordinate system is set up first. The x-axis is tangential to the wheel, the y-axis is perpendicular to the x-axis and points upward, and the z-axis points outward from the wheel. In this setup, the force on the wheel axle lies along the x-axis. The axle...

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Couple Moments and the Right-Hand Rule

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Equivalent Couples in Mechanical Systems

Equivalent couples in mechanical systems describe pairs of forces that create the same rotational effect on a rigid body. Two couples are equivalent when they have the same moment magnitude and act in the same direction. They then cause the same angular displacement or angular acceleration in the body. A couple is made of equal and opposite forces separated by a perpendicular distance. The moment of the couple depends on both the force size and the spacing between the forces. For example, one...

moment of a couple: problem solving

Torque from a Couple: Solving Forces

The moment of a couple is a key physics and engineering idea for finding torque, or rotational force. A couple is made of two equal and opposite forces that act on an object. When those forces work together, they create a twisting effect that can rotate the object. The moment of a couple is calculated by multiplying the size of one force by the perpendicular distance between the two lines of action. That distance is the shortest straight-line distance between the force directions. This gives...

system of forces and couples

Equivalent Force and Moment Systems

Structural systems often experience several forces and couple moments at the same time. Engineers simplify these loading cases by replacing them with an equivalent force and moment system at a chosen point O. This makes the analysis easier while keeping the same external effects on the member. A key idea in this process is the principle of transmissibility. A force is a sliding vector, which means it can be moved anywhere along its line of action without changing how the body behaves...

simplification of a force and couple system i

simplification-of-a-force-and-couple-system-i

Resultant Forces in Force and Couple Systems

Force and couple systems can be reduced to a simpler equivalent system. This makes rigid body analysis easier because the external effect of the forces is captured with fewer elements. When the resultant force and resultant couple moment have perpendicular lines of action, the system may be reduced further to a single resultant force along a new line of action. Three common force systems can be simplified this way: concurrent, coplanar, and parallel. A concurrent force system has lines of...

simplification of a force and couple system: ii

simplification-of-a-force-and-couple-system-ii

Resultant Force and Wrench Motion

Resultant force and resultant couple moment describe how several forces and moments act together on a rigid body in three dimensions. When many forces act on an object, they can be replaced by one equivalent force. The moments from those forces can also be combined into one equivalent couple moment. In some cases, the resultant force and the resultant couple moment are not mutually perpendicular. That means the angle between them is not 90 degrees. To study this situation, the couple moment is...

distributed loads

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Load distribution helps engineers describe how force spreads across a surface or structure. A distributed load is a continuous force per unit area, rather than a single point force. It is a common idea in many practical situations. A bookshelf full of stacked books is one simple example. The weight of the books is spread along the shelf, so the pressure changes across the surface. This pressure is measured in Newtons per square meter, or Pascals. A hydroelectric dam gives another clear...

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Structural engineering often requires finding the resultant load on a beam with changing force patterns. Here, a beam of length L carries a varying load made from parabolic and trapezoidal load distributions along the x-axis. The goal is to find the total load, where it acts, and the centroid of the combined area so the beam’s response can be predicted. The parabolic load is handled first. A small force element, dR, acts over a short distance, dx, and these elements are integrated across the...

distributed loads: problem solving

Equivalent Resultant Loads on Beams

Equivalent resultant loads on beams can be found by breaking a distributed load into smaller regions. This makes the beam easier to analyze. Each region can then be handled on its own before the results are combined. The magnitude of the equivalent resultant load for each region comes from the area of that region. In this setting, area represents the force applied by the distributed load over that part of the beam. After finding the force for each region, the position of each resultant load...

key terms and concepts

Learn how to solve moment of a force problems using free-body diagrams, perpendicular distance, net moment, and rotation direction for high school physics.

Calculating Force Moments in Problem Solving | 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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Calculating Force Moments in Problem Solving

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