Classical mechanicsLesson 1 of 1 in the course Newton's Laws of Motion

Newton’s Third Law, Motion, and System Boundaries

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

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Summary

This lesson introduces Newton’s third law and explains what it means for forces to occur in pairs. Understanding the law helps explain how objects interact, from a hand pushing on a wall to a swimmer moving away from a pool’s edge.

The subscripts identify which object exerts the force and which object experiences it. For example, FABF_{AB} means the force exerted by A on B, while FBAF_{BA} means the force exerted by B on A.

All flashcards

State Newton’s third law.
When one body exerts a force on another, the second simultaneously exerts an equal-magnitude, opposite-direction force on the first.
What does FABF_{AB} mean?
The force exerted by body A on body B.
What should a free-body diagram show?
The forces acting on the selected object or system.
How are internal and external forces distinguished?
Internal forces act between parts within the chosen system; external forces come from outside it.
What does a scale measure in general?
The contact force it exerts on the object, not necessarily the object's weight.

Quiz questions

  1. What distinguishes the two forces in a Newton’s third-law pair?Answer: They have equal magnitudes, opposite directions, and act on different bodies.
  2. A hand pushes on a wall. Which interaction force acts on the hand?Answer: The force the wall exerts on the hand.
  3. A runner pushes backward on the ground. What horizontal force from the ground can propel the runner forward?Answer: A forward friction force on the runner.
  4. Why can a rocket produce thrust in a vacuum?Answer: It interacts with its expelled exhaust gases, which exert a forward force on it.
  5. For a system containing both a professor and a cart, how is the professor’s force on the cart classified?Answer: It is an internal force between parts of the system.

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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