Competency #1
Answer and Explanation
Before we get started, let us state Newton's 3rd Law:
When object A exerts a force on object B, object B exerts an equal force on object A.
Note that 3rd Law pairs CANNOT both be acting on the same object!
Now, let's list all of the forces that act on the book at any point during the scenario:
- Force of gravity, FG
- Support force, Fsupport
- Force of friction, Ffriction
- Force of hand (could also be called an applied force), Fhand
We have identified four forces acting on the book. Now, we need to find the 3rd law pair for each.
- Force of gravity - To be more specific, it is the force of gravity of the earth pulling down on the book. If the earth is pulling down on the book, the book must be pulling up on the earth.
- Support force - If the table is pushing up on the book, the book must be pushing down on the table.
- Force of friction - Since there is a frictional force of the table pushing on the book to the right, there must be a frictional force of the book pushing on the table to the left.
- Force of hand - While the hand is pushing the book to the left, the book is pushing the hand to the right.
| Summary of 3rd Law Pairs in this Scenario: | |
|---|---|
| Force of gravity of earth pulling down on book | Force of gravity of book pulling up on earth |
| Table pushes up on book | Book pushes down on the table |
| Friction of table pushing on book to the right | Friction of book pushing on the table to the left |
| Force of hand pushing on book to the left | Force of book pushing on hand to the right |
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