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Newton's Third Law of Motion: The Physics of Rockets

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Newton's Third Law

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Newton's Third Law of Motion: The Physics of Rockets

Physical Science

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You will explore Newton’s third law using the example of the working principles of rockets.

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Now You Know

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

  • Explain that when gas is exerted from a rocket in one direction, the gas will push the rocket in the opposite direction.
  • Explain that after a force has been exerted on an object in frictionless space, the object will continue moving with the same speed and direction.
  • Explain that objects change velocity only when an external net force is applied.
  • Explain Newton’s third law.

Everything You'll Have Covered

Sir Isaac Newton was born in England, and investigated many areas, 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, "For every action, there is an equal and opposite reaction."

Understanding how rockets work requires an understanding of all of three of Newton's laws of motion, but especially the first and third laws. A rocket will continue its state of motion or stay at rest when gas is not being expelled. However, when the rocket pushes gas backwardbackwards with some force, the gas will push the rocket and the astronaut forward with the same amount of force. This allows the astronaut to accelerate, or change his velocity.

As soon as the rocket is turned off so that gas is no longer being forced out, the astronaut will continue moving with that speed until another force acts upon him. Since there is no friction in space, there is no reason for the astronaut to change his speed unless he is engaging the rockets. In order to stop the motion of the astronaut, gas must be expelled so that the gas is pushing on the rocket in a direction opposite to its motion.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Learners should be familiar with forces and Newton's first law.
Course Physical Science
Type of Tutorial Concept Development
Key Vocabulary action, reaction, Newton’s third law