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Section learning objectives

By the end of this section, you will be able to:

  • Describe Newton's Third Law both verbally and mathematically
  • Use Newton's Third Law to solve problems

The Learning Objectives in this section will help your students master the following TEKS:

  • (4) Science concepts. The student knows and applies the laws governing motion in a variety of situations. The student is expected to:
    • (D) : calculate the effect of forces on objects, including the law of inertia, the relationship between force and acceleration, and the nature of force pairs between objects
Section key terms
Newton's third law of motion normal force tension thrust

Describing newton’s third law of motion

[BL] [OL] Review Newton’s First and Second Laws.
[AL] Start a discussion about action and reaction by giving examples. Introduce the concepts of systems and systems of interest. Explain how forces can be classified as internal or external to the system of interest. Give examples of systems. Ask students which forces are internal and which are external in each scenario.

If you have ever stubbed your toe, you have noticed that although your toe initiates the impact, the surface that you stub it on exerts a force back on your toe. Although the first thought that crosses your mind is probably “ouch, that hurt” rather than “this is a great example of Newton’s third law,” both statements are true.

This is exactly what happens whenever one object exerts a force on another—each object experiences a force that is the same strength as the force acting on the other object but that acts in the opposite direction. Everyday experiences, such as stubbing a toe or throwing a ball, are all perfect examples of Newton’s third law in action.

Newton’s third law of motion states that, whenever a first object exerts a force on a second object, the first object experiences a force equal in magnitude but opposite in direction to the force that it exerts

Newton’s third law of motion tells us that forces always occur in pairs, and one object cannot exert a force on another without experiencing the same strength force in return. We sometimes refer to these force pairs as “action-reaction” pairs, where the force exerted is the action, and the force experienced in return is the reaction (although which is which depends on your point of view).

Newton’s third law is useful for figuring out which forces are external to a system. Recall that identifying external forces is important when setting up a problem, because the external forces must be added together to find the net force.

Mass and weight

In this activity you will use a scale to investigate mass and weight.

...
  • 1 scale
  • 1 table
  • Organisms: This lab uses organisms that can pass on diseases.
  1. What do bathroom scales measure?
  2. When you stand on a bathroom scale, what happens to the scale? It depresses slightly. The scale contains springs that compress in proportion to your weight—similar to rubber bands expanding when pulled. Newton’s 1st law: Fnet = 0 or (or ∑F = 01)
  3. The springs provide a measure of your weight (provided you are not accelerating). This is a force in newtons (or pounds). In most countries, the measurement is now divided by 9.80 to give a reading in kilograms, which are units of mass. The scale detects weight but is calibrated to display mass.
  4. If you went to the Moon and stood on your scale, would it detect the same “mass” as it did on Earth? How about: a. mars b. venus c.saturn
Table 3: Snap lab testing table
Planet Mass Weight (Fnet = 0 or (or ∑F = 01)
Mars 12kg 5N
Venus 20kg 90N
Saturn 1kg 13N

Explain that, even though a scale gives a mass, it actually measures weight. Scales are calibrated to show the correct mass on Earth. They would give different results on the Moon, because the force of gravity is weaker on the Moon.

Practice Key Terms 4

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Source:  OpenStax, Updated tutor hs physics content - legacy. OpenStax CNX. Mar 16, 2015 Download for free at https://legacy.cnx.org/content/col11768/1.4
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