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Part A Learning Goal: 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. If you haven't learned about momentum, don't worry; for now this is just a statement about the origin of forces. 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 mass. Every force has one and only one 3rd law pair force. O true O false Submit Request Answer Part B The two forces in each pair act in opposite directions. O true O false Submit Request Answer Part C The two forces in each pair can either both act on the same body or they can act on different bodies. O true

Part A
Learning Goal:
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. If you haven't learned about
momentum, don't worry; for now this is just a
statement about the origin of forces.
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 mass.
Every force has one and only one 3rd law pair force.
O true
O false
Submit
Request Answer
Part B
The two forces in each pair act in opposite directions.
O true
O false
Submit
Request Answer
Part C
The two forces in each pair can either both act on the same body or they can act on different bodies.
O true

Added by Sara A.

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