Newton's third law of motion

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  1. Introduction to Newton's third law
  2. What is Newton's third law?
  3. Identifying forces in action-reaction force pair
  1. Understanding Newton's third law and free body diagrams

    A rock sits on a table. Draw complete free body diagrams for the rock and the table and highlight the action-reaction pair that includes the rock pushing down on the table.

    1. Calculations with Newton's second and third laws
      1. An 85.0 kg fireman slides down a pole with an acceleration of 2.85 m/s2m/s^{2}. Find the action-reaction force pair that involves the pole and calculate these forces.
      2. Two ice skaters stand on a frictionless ice rink and face each other, each holding the end of a rope. 50.0 kg skater A pulls on the rope, pulling 40.0 kg skater B with a force of 25.0 N.

        i. Draw a free body diagram and find the acceleration of skater B.

        ii. When skater A pulls on skater B, does skater A accelerate? If so, draw a free body diagram and calculate the acceleration.

    Topic Notes
    In this lesson, we will learn:
    • Newton's third law of motion
    • Explaining physical phenomena using Newton's third law
    • Calculations with Newton's second and third laws


    • Newton's third law: when one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
      • Also stated: "for every action, there is an equal and opposite reaction."
    • Each force in action-reaction force pair is located on a different object in the pair. Each force in action-reaction force pair also has the same "type."
      • Example: if a wooden block slides across a floor, the floor exerts a force of friction on the block and the block exerts a force of friction that is equal in magnitude but opposite in direction on the floor. The forces in this pair are on different objects (one acts on the block, and the other acts on the floor), and both are the same type of force (friction).
    Newton's Third Law

    For object A exerting a force on object B:

    FAonB=FBonA\vec{F}_{A on B} = - \vec{F}_{B on A}

    FAonB:\vec{F}_{A on B}: force A is exerting on B, in newtons (N)

    FBonA:\vec{F}_{B on A}: force B is exerting on A, in newtons (N)