Part A Every force has one and only one 3rd law pair force. Every force has one and only one 3rd law pair force. true false     Part B The two forces in each pair are exerted in opposite directions. The two forces in each pair are exerted in opposite directions. true false     Part C The two forces in each pair can either both be exerted on the same body or they can be exerted on different bodies. The two forces in each pair can either both be exerted on the same body or they can be exerted on different bodies. true false

To understand Newton's 3rd law, which states that a physical interaction always generates a pair of forces on the two interacting bodies.

In Principia, Newton wrote:To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

(translation by Cajori)

The phrase after the colon (often omitted from textbooks) makes it clear that this is a statement about the nature of force. The central idea is that physical interactions (e.g., due to gravity, bodies touching, or electric forces) cause forces to arise between pairs of bodies. Each pairwise interaction produces a pair of opposite forces, one acting on each body. In summary, each physical interaction between two bodies generates a pair of forces. Whatever the physical cause of the interaction, the force on body A from body B is equal in magnitude and opposite in direction to the force on body B from body A.

Incidentally, Newton states that the word "action" denotes both (a) the force due to an interaction and (b) the changes in momentum that it imparts to the two interacting bodies. Ultimately, Newton's 3rd law is a direct consequence of the law of conservation of momentum.

Mark each of the following statements as true or false. If a statement refers to "two bodies" interacting via some force, you are not to assume that these two bodies have the same inertia.

 

Every force has one and only one 3rd law pair force.

 

 

The two forces in each pair are exerted in opposite directions.

 

 

The two forces in each pair can either both be exerted on the same body or they can be exerted on different bodies.

 

 

The two forces in each pair may have different physical origins (for instance, one of the forces could be due to gravity, and its pair force could be due to friction or electric charge).

 

 

The two forces of a 3rd law pair are always exerted on different bodies.

 

 

Given that two bodies interact via some force, the accelerations of these two bodies have the same magnitude but opposite directions. (Assume no other forces act on either body.)

 

 

 

 

 

According to Newton's 3rd law, the force on the (smaller) moon due to the (larger) earth is