Classical mechanicsLesson 2 of 2 in the course Newton's Laws of Motion

Free-Body Diagrams: Components, Contact Forces, and Constraints

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

A free-body diagram is a focused picture of the external forces acting on one chosen object. It helps organize information before applying Newton’s laws. In this lesson, you will learn to isolate an object, identify forces that act on it, and represent those forces accurately.

A free-body diagram (FBD) represents one object separated from its surroundings, with arrows showing the external forces exerted on that object. The object may be drawn as a simple shape or, when its size and rotation are not important, as a point. The diagram is not a picture of the object’s motion; it is a model of the forces acting on it.

All flashcards

What is a free-body diagram?
A diagram that isolates one object and shows all external forces acting on it.
What is the weight force, and which way does it point?
Weight is the gravitational force on an object. Near Earth’s surface it points vertically downward and has magnitude w=mgw=mg.
What does a normal force represent?
It is a contact force exerted by a surface, directed perpendicular to the surface and away from it.
Why are action-reaction forces not both drawn on one object's FBD?
The two forces in a Newton’s third-law pair act on different objects. Each force belongs on the diagram for the object it acts on.
Should acceleration or net force be drawn as an interaction force?
No. Acceleration is not a force, and net force is the vector sum of the forces already drawn, not an additional interaction.

Quiz questions

  1. What does a free-body diagram represent?Answer: One selected object and all external forces acting on it.
  2. Which direction does an object's weight point near Earth's surface?Answer: Vertically downward, toward Earth.
  3. A book rests on a table; neglect air effects. Which forces belong on the book's free-body diagram?Answer: The book's weight and the table's normal force.
  4. Why should the net force not be drawn as an additional force arrow on a free-body diagram?Answer: It is the vector sum of the forces already shown, not a separate interaction force.
  5. A taut rope pulls a box. How should the rope's force on the box be represented?Answer: As a tension vector along the rope, directed away from the box.

Based on University Physics Volume 1 by William Moebs, Samuel J. Ling, Jeff Sanny (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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