# Newton's Laws: Book-Table-Earth

1. Oct 15, 2013

### mjm3

1. The problem statement, all variables and given/known data
A book is sitting on a table, which is sitting on the Earth. The book has a weight of 10 N and the
table has a weight of 20 N as labeled on the diagram.
a. Find all contact forces, F12. (For example, “FEB” would mean “Force of Earth ON
Book”)
b. Next to each, tell me which of Newton’s laws allowed you to make your
conclusion. (2nd or 3rd)

2. Relevant equations

3. The attempt at a solution
New to free body diagrams. I think I was able to name the contact forces for this concept problem.

F(bt) downward F(tb) upward. This would be an example of the third law.
F(te) downward F(et) upward. Also an example of the law.

If W(b) downward is 10 N then F(tb) upward = 10 N also

If W(t) downward is 20N, then F(bt) downward is 10N, which should make F(et) = 30N

2. Oct 15, 2013

### HallsofIvy

Staff Emeritus
"Force of earth on book" is NOT a "contact force" because the earth is not in contact with the book. The "contact forces" are "book on table", "table on book", "table on earth" and "earth on table". Of course the "table on earth" contact force depends upon the total weight of the table and book.