5.13
Diagrammes de corps libre : exemples
Résoudre des problèmes qui impliquent des forces est facile en utilisant des diagrammes de corps libre. Un diagramme de corps libre est un croquis mon…
Les diagrammes à corps libre sont des représentations graphiques de diverses forces exercées sur un objet. Cela peut être compris à l’aide de quelques exemples.
Le schéma du corps libre d’une lampe suspendue à une chaîne peut être représenté par un point avec deux forces : la force gravitationnelle et la tension dans la chaîne.
En prenant un autre exemple, esquissez un schéma de corps libre pour un chariot transportant une grande boîte. Ici, il y a deux systèmes d’intérêt, la boîte et le chariot ; Il est donc nécessaire de dessiner des diagrammes distincts pour chacun.
Les forces agissant sur la boîte sont la force gravitationnelle due à sa masse, la force normale et la force de frottement exercée par le chariot sur la boîte.
Pour le chariot, la force gravitationnelle, la force normale nette due au sol, une force normale due à la boîte sur le chariot, la force de frottement nette exercée par le sol et la boîte sur le chariot, et une poussée appliquée sur la poignée, agissent toutes ensemble.
Q1: What is a free-body diagram and why is it useful?
A free-body diagram is a sketch showing all external forces acting on an isolated object or system, represented as a single point with force vectors extending outward. It simplifies force analysis by displaying only forces originating outside the object, making it easier to solve problems involving internal and external forces and apply Newton's laws to predict motion.
Q2: How do you represent forces in a free-body diagram?
Forces are represented as vectors extending outward from the free body point. Each vector's direction indicates the force direction, and its length represents the force magnitude. Directions are labeled with plus or minus signs, with conventions like rightward as positive. This visual representation helps identify which forces act on an object and their relative magnitudes.
Q3: Why do you need separate free-body diagrams for different objects in a system?
When analyzing a system with multiple objects like a cart carrying a box, separate diagrams isolate each object's external forces. The box experiences gravitational force, normal force, and friction from the cart, while the cart experiences its own weight, ground forces, and applied push. Separating them prevents confusion between internal forces between objects and external forces acting on each.
Q4: What forces act on an object hanging from a chain?
An object hanging from a chain experiences two forces: gravitational force pulling downward due to its mass and tension in the chain pulling upward. These opposing forces can be represented in a free-body diagram as vectors extending from a point, showing how the tension must equal the weight for the object to remain in equilibrium.
Q5: How do you handle force components on an inclined surface?
On an inclined surface like a slope, the normal force is drawn perpendicular to the slope surface and friction parallel to it. The weight vector has components along both axes: one perpendicular to the slope and one parallel to it. This decomposition allows you to analyze forces in directions relevant to the surface geometry and motion constraints.
Q6: When can vertical forces be ignored in a free-body diagram?
Vertical forces can be ignored when there is no vertical acceleration, meaning upward and downward forces cancel each other. For example, a rocket sled on a horizontal surface has normal force balancing weight, leaving only horizontal forces to analyze. This simplification reduces the problem to one dimension, making calculations more straightforward.
Q7: What forces must be included when drawing a free-body diagram?
Include only external forces acting on the object from outside sources: gravitational force, normal force, friction, tension, applied pushes, and thrust. Exclude internal forces between components within the system. For a person sitting on a chair, show only the chair's normal force upward and gravitational force downward, not internal muscle forces.