Journal Entry 2/1 Continued

Newton's Third Law: F a on b = - F b on a

1. You are seated on a bench. The earth exerts a force on you, and the bench exerts a force on you. Do these two forces represent a Newton’s Third Law force pair? Explain.

In this situation, both forces are being exerted on me, which means they violate Newton's Third Law and cannot be force pairs.

2. A pen sits on a desk. The desk exerts a force on the pen, and the pen exerts a force on the desk. Do these two forces represent a Newton’s Third Law force pair? Explain.

In this case, the pen is object A and the desk is object B. Both are exerting a opposite force on the other, making them a Newton's Third Law force pair.

3. You pull on a rope. The rope pulls on a wagon. Do these two forces represent a Newton’s Third Law force pair? Explain.

Although there are two forces being exerted here, they are being exerted on different objects. Object A (me) is pulling on object B (rope). Object B (rope) however is pulling on Object C (wagon). These forces do not support Newton's Third Law.

4. A book slides across a tabletop. The book exerts a downward force on the table, and the table exerts a frictional force on the book opposite the direction of its motion. Do these two forces represent a Newton’s Third Law force pair? Explain.

Although there are two forces here, the book's downward force on the table and the frictional force of the table on the book, they do not correlate because one is a vertical force and the other is a horizontal force.

In my original post, I continually remarked that an object could not be moving without an unbalanced force. This is a false statement. Through recent lessons I have to come to realize that an unbalanced force is only necessary for initial movement. Once an object is in motion, there does not need to be an unbalanced force. For example, after a bowling ball is rolled and it reaches a constant speed, there is no unbalanced force on it, but it is still moving.