Name: free-body diagram for two-block motion
Uploaded: 2025-05-22T09:06:02.000Z
Duration: 1 min 22 s
Channel: JoVE

K12 Standards

Next Generation Science Standards

High School

Physical Science

motion-and-stability-forces-and-interactions

Motion and Stability: Forces and Interactions

HS-PS2-3

Steps for Free-Body Diagram

Considérons un système de deux blocs couplés par une corde sans masse sur une poulie. Le bloc 1 glisse sur une table tirée par le bloc 2 lorsqu’il tombe sous l’effet de la gravité. 
L’accélération du système peut être dérivée en calculant la force nette sur le système.
Un schéma à corps libre pour ce système peut être dessiné en représentant les deux blocs avec leur contour. Ici, les blocs sont considérés comme isolés.
Les forces externes qui agissent sur eux sont identifiées et représentées par des flèches vectorielles.
Les forces sur le bloc 1 sont son poids, la force normale, la force de tension due à la corde et la force de friction opposée à la force de tension à l’origine du mouvement.
De même, les forces agissant sur le bloc 2 sont son poids et la force de tension.
En appliquant la deuxième loi du mouvement de Newton aux blocs, une équation de la force nette peut être dérivée.
En utilisant l’opération algébrique et en réarrangeant les termes, l’intensité de l’accélération du système peut être obtenue.

<ol>
	<li>Hibbeler, R.C. (2016). Génie mécanique \u2012 Statique et dynamique. Hoboken, New Jersey : Pearson Prentice Hall. page 89</li>
	<li>Bière, F.P. ; Johnston, E.R. ; Mazurek, D.F ; Cromwell, P.J. et Self, B.P. (2019). Mécanique vectorielle pour les ingénieurs \u2012 Statique et dynamique. New York : McGraw-Hill. p. 171 et 172</li>
</ol>

Étapes pour un diagramme des forces

Lorsqu'il s'agit d'étudier le comportement des objets en mécanique, l'un des outils les plus importants disponibles est le diagramme des forces. Considérons un exemple simple d'un système de deux blocs couplés par un fil sans masse sur une poulie sans friction. Le bloc 1 glisse sur une table tiré par le bloc 2 tandis que le bloc 2 tombe sous l'effet de la gravité. Pour trouver l'accélération du système, il est tout d'abord nécessaire de calculer la force nette agissant sur le système. Pour cela, un diagramme des forces peut être créé pour représenter les deux blocs, qui sont considérés comme un seul système isolé de l'environnement. Les forces externes sont alors identifiées et représentées par des flèches vectorielles sur le diagramme. En regardant le bloc 1, plusieurs forces agissent sur lui. Celles-ci incluent son poids, la force normale, la force de tension due à la corde et la force de friction qui s'oppose à la force de tension, entraînant ainsi le mouvement. Pour le bloc 2, son poids et les forces de tension sont les seules forces agissant sur lui. Une fois le diagramme des forces créé, il est possible d'appliquer la deuxième loi de Newton du mouvement aux blocs. En faisant cela, une équation pour la force nette agissant sur le système peut être dérivée. Cette équation inclura les masses des blocs, l'accélération du système et les forces agissant sur chaque bloc. Avec l'équation en main, il est alors possible d'utiliser des opérations algébriques pour réarranger les termes et résoudre la magnitude de l'accélération du système. Le résultat sera une valeur numérique qui représente la façon dont le système se comportera sous l'effet des forces données. L'importance d'un diagramme des forces dans cette situation ne peut être surestimée. Il fournit une représentation claire et concise des forces en jeu, permettant ainsi un calcul plus facile et plus précis de l'accélération du système. Sans cet outil, il serait beaucoup plus difficile de comprendre et d'analyser le comportement des blocs et les forces agissant sur eux.

Lorsqu’il s’agit d’étudier le comportement des objets en mécanique, l’un des outils les plus importants disponibles est le diagramme à corps libre. Considérons un simple ...

Education

Enseignement

JoVE Core

Mechanical Engineering

Génie mécanique

Unit 1

Unité 1

Equilibrium of a Particle

Équilibre d'une particule

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

free-body diagram for two-block motion

A free-body diagram helps analyze two-block motion in mechanics. It shows the forces acting on a system so the net force can be found and the acceleration can be calculated.

The example in this lesson uses two blocks linked by a massless string over a frictionless pulley. Block 1 moves across a table while block 2 falls under gravity. The two blocks are treated as one isolated system.

External forces are then identified and drawn as vector arrows on the diagram. For block 1, the forces include weight, the normal force, tension from the rope, and friction that opposes the tension force. For block 2, the forces are its weight and the tension force.

After the free-body diagram is complete, Newton’s second law of motion can be applied to the blocks. This gives an equation for the net force on the system. The equation includes the masses of the blocks, the acceleration of the system, and the forces acting on each block.

With the equation set up, algebra can be used to rearrange the terms and solve for the magnitude of the acceleration. The result is a numerical value that describes how the system will move under the given forces. The diagram makes the force analysis clearer and more accurate.

Free-Body Diagram for Two-Block Motion

equilibrium conditions for a particle

Balanced Forces and Torque in Equilibrium

Equilibrium depends on balanced forces and balanced torque. An object in equilibrium is either at rest or moving with a constant velocity. These two cases are called static equilibrium and dynamic equilibrium. Static equilibrium means the object is not moving. Dynamic equilibrium means the object is moving, but its velocity stays constant. In both cases, the forces acting on the object must balance each other. Different forces can add, subtract, or cancel each other out. When the vector sum...

free-body diagram

Free-Body Diagrams for Force Analysis

Free-body diagrams help analyze the forces acting on an object in mechanics. They give a simple picture of a stationary or moving object and show why it moves the way it does. By turning a complex situation into a clear sketch, they make it easier to write the equation needed to solve the problem. To draw a free-body diagram, first choose the object or system you want to study. Then isolate it from its surroundings and draw it as a dot or a simple outline. Add arrows for every important...

steps for free-body diagram

A free-body diagram helps analyze two-block motion in mechanics. It shows the forces acting on a system so the net force can be found and the acceleration can be calculated. The example in this lesson uses two blocks linked by a massless string over a frictionless pulley. Block 1 moves across a table while block 2 falls under gravity. The two blocks are treated as one isolated system. External forces are then identified and drawn as vector arrows on the diagram. For block 1, the forces...

free-body diagrams: problem solving

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Free-body diagrams help show the forces acting on an object on an inclined plane. A free-body diagram is a simple drawing that focuses only on the object or system being studied. It leaves out extra details so the force problem is easier to analyze and solve. For an example, a 10 kg block is released on a ramp that is 30° to the horizontal. The coefficient of friction is 0.2. The diagram for this block includes the weight from gravity, the normal force, and the frictional force. These are the...

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Coplanar forces act on an object in the same plane. To analyze this kind of system, each force is resolved into x and y components using Cartesian vector form and vector algebra. This makes it possible to test whether the object is in equilibrium. A coplanar system is in equilibrium when the resultant force is zero. That means the sum of the forces in each direction must also be zero. If the forces do not balance, the object is not in equilibrium and will accelerate in the direction of the net...

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Types of Equilibrium in Forces

Equilibrium in physics describes how forces act on a body so it stays still or moves at a constant speed. In two-dimensional force systems, equilibrium can be grouped into several categories based on how the forces are arranged. Collinear equilibrium is the simplest case. The forces act along one straight line, so only one force equation is needed in the direction of those forces. The other equations are already satisfied. Coplanar concurrent equilibrium is another category. In this case, all...

key terms and concepts

Learn how a free-body diagram is used to analyze two blocks connected by a string and pulley, identify forces, and solve for acceleration.

Free-Body Diagram for Two-Block Motion | JoVE K12

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HS-PS2-6

Free-Body Diagram for Two-Block Motion

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