Newton's Third Law of Motion (page 2)

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Author: Janice VanCleave


  1. Every force on an object causes the object to be compressed to some degree. Design an experiment to demonstrate that an object compresses until the action-reaction forces are equal. For example, fill a 3-ounce (90-ml) cup with coins. Lay two similar-size books about 10 inches (25 cm) apart on a table. Support the ends of a thin, flexible, plastic ruler on the books. Set the cup of coins in the center of the ruler. Science Fair Hint: Make a diagram showing the compression of the ruler, the action-reaction pairs of forces, such as in Figure 54.3. Three legends describing the force pairs of the books and the cup can be added to the drawing. For example:
  2. Action-Reaction

  3. Unaccompanied forces do not exist. Since all forces are in pairs of equal strength, acting in opposite directions, and acting on different objects, what causes motion? A resultant force, is the single force that has the same effect as the sum of two or more forces acting simultaneously on one object. When forces simultaneously act on an object and the resultant force is zero, the forces are said to be balanced forces and produce no acceleration. When the resultant force of a group of forces acting simultaneously on an object is not equal to zero, the forces are said to be unbalanced forces. Newton's first law of motion explains that an unbalanced force acting on an object is needed to cause acceleration. In this law, the resultant force is the net force (the sum of all forces acting simultaneously on an object).
  4. Design an experiment to demonstrate that a pair of action-reaction forces are unbalanced because they act on different objects. One way is with two identical balloons. Inflate one of the balloons and tie a knot in its open end. Lay the balloon on a table and observe any motion of the balloon. Science Fair Hint: Prepare a diagram representing the action-reaction forces for the open and closed balloons, such as in Figure 54.4. Write the calculations for determining the net force of the gas inside the closed balloon on the balloon, represented by forces A as well as the net force of the balloon on the gas inside the balloon, represented by forces B. The equation for net force is:

    The calculation for determining the net force of the gas on the balloon in the closed balloon is:

    The equation representing the net forces of the gas acting on the open balloon is:

    In the closed balloon, the net force is zero, therefore there is no unbalanced force and thus no motion. In the open balloon, the net force is equal to force A1↑. Thus there is an unbalanced force in the up direction causing the balloon to move upward. Use the net force equation to calculate the net force of the balloon on the gas inside the open and closed balloons to determine why the gas moves out of the open balloon.


Get the Facts

  1. The use of steam as a source of power can be traced back to a toy invented by a Greek engineer named Hero of Alexandria (20?–62?). This toy turned as a result of action-reaction forces. For information about the construction of Hero's toy and how the action-reaction forces were produced, see Struan Reid's and Patricia Fara's Inventors (Tulsa, Okla: EDC, 1994), p. 10.
  2. A simple Hero's engine can be made from a soda can. For information about building this simple Hero's engine and making predictions about its movement see Robert Ehrlich's Why Toast Lands Jelly-Side Down (Princeton, N.J.: Princeton University Press, 1997), pp. 69–71.
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