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Newton's Third Law

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Newton's Third Law


Key Equations



Example 1









Watch this Explanation


Time for Practice

  1. You are standing on a bathroom scale. Can you reduce your weight by pulling up on your shoes? (Try it.)
  2. A VW Bug hits a huge truck head-on. Each vehicle was initially going 50 MPH.
    1. Which vehicle experiences the greater force?
    2. Which experiences the greater acceleration? Explain briefly.
  3. You and your friend are standing on identical skateboards with an industrial-strength compressed spring in between you. After the spring is released, it falls straight to the ground and the two of you fly apart.
    1. If you have identical masses, who travels farther?
    2. If your friend has a bigger mass who goes farther?
    3. If your friend has a bigger mass who feels the larger force?
    4. If you guys have identical masses, even if you push on the spring, why isn’t it possible to go further than your friend?
  4. Analyze the situation shown here with a big kid pulling a little kid in a wagon. You’ll notice that there are a lot of different forces acting on the system. Let’s think about what happens the moment the sled begins to move.
    1. First, draw the free body diagram of the big kid. Include all the forces you can think of, including friction. Then do the same for the little kid.
    2. Identify all third law pairs. Decide which forces act on the two body system and which are extraneous.
    3. Explain what conditions would make it possible for the two-body system to move forward.


Discuss in class.

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