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Newton's Third Law of Motion

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Newton's Third Law

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Lesson Focus

Newton's Third Law of Motion


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Students learn about action-reaction forces.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Describe Newton’s third law of motion.
  • Give examples of Newton’s third law in daily life.
  • Explain that when a spring is placed between two identical trucks and released, the trucks will move in opposite directions.
  • Explain that when a spring is placed between two identical trucks and released, the trucks will move the same distance in the same amount of time.

Everything You'll Have Covered

Sir Isaac Newton was born in England, and investigated many areas of study, including mathematics, physics, and astronomy. He is probably best known for his three laws of motion.

Newton's first law states, "An object in motion will continue its motion unless acted upon by an unbalanced force. An object at rest will remain at rest unless acted upon by an unbalanced force." Newton's second law goes on to state that an object's acceleration depends on the net force acting on it and the mass of the object. This law is generally represented with the mathematical relationship: F = ma. Newton's third law states, "Whenever an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object." In this third law, the focus is on the action-reaction force pair.

When identifying action-reaction force pairs, it is important for learners to realize that this law does not negate Newton's first law of motion, because the action force acts on one object, and the reaction force acts on the second object. Either of the objects can accelerate as long as there are unbalanced forces acting on the object.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with acceleration, forces, Newton’s first law, and Newton’s second law.
Course Physics
Type of Tutorial Concept Development
Key Vocabulary acceleration, action, force