Reflection: Lesson Planning Newton's 3rd Law Demonstration - Section 3: Student Demonstration

 

Newton's 3rd law is, in my opinion, the easiest to teach. This simplicity is not without risk of introducing a misconception. Kids understand that relationship between an act and a consequence. The version of Newton's 3rd Law that I teach is:

For every action there is an equal and opposite reaction.

I would recommend using a rocket blasting off as the example to teach this law. A rocket produces downward thrust, which results in the upward direction of travel. The force of the thrust influences the acceleration of the rocket. The rocket's downward thrust is equal to and in the opposite direction of the rocket's upward acceleration.

Students sometimes get confused when Newton's 3rd Law is applied to contact forces. If a boxer punches an opponent and the opponent flies backward the action/reaction may be equal but they don't appear opposite. The punch (action) is in the same direction as the staggering opponent (reaction). This example is really Newton's 2nd Law, in that the acceleration of an object (staggering opponent) is in the same direction as the applied force (punch).

A nice way to end this series of demonstrations (Newton's Laws 1-3) is to provide an example where all three laws are used to explain the simple act of walking.

  1. Before you start walking you are an object at rest that needs a force (pushing on the floor with your foot) to become an object in motion. Once in motion, a force is required to stop walking, such as gravity and/or friction. 
  2. Your forward motion (acceleration) is based upon how hard you are pushing with your foot on the ground (force) and how much you weigh (m). The harder you push, the faster you go. If you were to pick another student up while you are walking you would need to push harder on the ground with your foot if you wanted to maintain your current speed, otherwise you would slow down.
  3. When you push against the ground with your foot towards your rear an opposite reaction of you moving forward is produced. Your forward speed is directly related to how hard your foot pushes.

  Avoid misconceptions
  Lesson Planning: Avoid misconceptions
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Newton's 3rd Law Demonstration

Unit 6: Forces
Lesson 12 of 12

Objective: Students will be able to observe Newton's 3rd Law of Motion as it applies to their physical world.

Big Idea: Newton's 3rd Law is not always intuitive to students. Providing a real world example helps them to understand the physical environment they live in.

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