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A string is tied to a book and pulled lightly as shown at right. The book remains in contact with the table and does not move.
a. Draw a free-body diagram for the book. Label each of the forces exerted on the book as you were instructed in the tutorial Forces. String
b. How do the forces exerted on the book in this case compare to the forces exerted on the book when the string is not present? List any forces that are the same (i.e., same type of force, same direction, and same magnitude) in both cases. Make a separate list of forces that change (or are not present) when the string is pulled.
If a book is located on a table, how many forces should be shown in a free-body diagram of the book? Describe them.
In this problem, we observe the simplest form of a free-body diagram. When a book is placed on the table, it has the force of gravity acting on it, applied to the book's center of mass. Consequentially, the book pushes the table with its weight, which exists due to gravity and the book's mass.
What cancels out the force of gravity on the book is the reaction, normal force. It is directed upwards and is applied to the surface of the book, the one which is in contact with the table. Since the book is not being pushed along any other axis, there are no more forces acting on the book.
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