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Newton’s Third Law: All Forces Come in Pairs

Warm-Up

You’re going to be concentrating on forces today, so let’s review a little about them.

What is the standard unit of force?_____

What is the symbol used to represent “force” in an equation?_____

Is force a vector or a scalar or neither?_____

What are three different kinds or types of forces?______

“Net force” is not a type of force. How is “net force” different from the types of forces you listed above?

______

Force Pairs

Every force is the result of an interaction between two objects, and these interactions always result in two forces, one force on each object. You can’t have one force alone, by itself. Isaac Newton described this principle, and it was important enough to become the third of his three “Laws of Motion.” Therefore, these force pairs are sometimes called “third-law pairs.”

To be a “third-law pair” of forces, the following must be true about the pair of forces:

•They must be the same type of force. For example, if one is a normal force, the other must also be a normal force (not gravitational, or tension, or friction…).

•Each force must be acting on a different object. Each object acts on the other. The two forces in the pair cannot both be acting on the same object.

•The forces must be equal in magnitude (that is, the forces have the same strength).

•The forces act in opposite directions.

Example: Suppose you push a big crate across the floor at constant speed. The force diagram for the box is shown. /

Now consider the normal force you exert on the box. According to Newton’s Third Law, there must be another force that is paired up with it. Does one of the other forces on this diagram complete the pair? Look at the table below. For each force, put a check mark in the box if the condition is met (for example: FN, floor is the same type, so you can put a check mark in the first box under it as shown). If the condition is not met, mark the box with an “X” (for example: the direction of Fg is not opposite the direction of FN, person, so you can mark it “X” as shown). Finally, in the last row, answer “Yes” or “No.” To be “Yes,” then all the conditions must be correct.

Condition to be Third Law Pair: / FN, Floor / Ff / Fg
Same Type?
Acting on Different Object?
Equal Magnitude?
Opposite Direction? / X
Does this force complete a 3rd law pair with FN, person ?

So, what force does make a third law pair with the force FN, person ? Start with a description of the first force in the pair:

You push on the box with a normal force to the right.

Then write a description for the second force in the pair, a description that is similar in form, but different in important ways:

The ______pushes on ______with a ______force to the ______.