Video Transcript
Fill in the blanks. Imagine a book rests on a
table. If the force due to Earth on the
book is the action force, then the reaction force acts on blank by blank. Is it (A) the book, Earth; (B) the
table, Earth; (C) the table, the book; or (D) Earth, the book?
Let’s start by refreshing our
memories about action and reaction forces. We might remember hearing the
phrase “For every action, there is an equal and opposite reaction.” This is a summary of Newton’s third
law of motion. A more complete description is
this. For a pair of interacting objects A
and B, any force that object B exerts on object A is equal in magnitude and opposite
in direction to any force object A exerts on object B.
For example, think about a person
sitting on a floor. The person exerts a downward force
on the floor. And also the floor exerts an upward
force on the person in order to support them. So we could say that the action
force here is the person acting on the floor, and the reaction force is the floor
acting on the person. This is what we mean by action and
reaction forces. It’s important to note that the
forces that act on pairs of objects that interact can both be contact forces, like
how the person and the floor are touching, or they can both be forces that act at a
distance, like gravity.
Now, let’s get back to the question
at hand. We’re thinking about a book that’s
just resting on a table, not moving. We know that Earth exerts a
gravitational force on the book, and we’re told that this is the action force. This means that the reaction force
must also be a gravitational force. In this case, we know that the
reaction force must be the gravitational force of the book acting on Earth. This is a good hint that answer
option (D) is correct.
Now, it might not be immediately
clear that something as light as a book could exert a gravitational force on
something as massive as a planet. But we need to remember that any
two objects with mass exert gravitational force on each other, whether it’s obvious
to an observer or not.
We can also be confident in our
answer because Newton’s third law, and thus the principle of action–reaction forces,
applies to a pair of interacting objects. That means two objects at a
time. Looking back at this description of
Newton’s third law, which uses objects A and B as examples, notice that it makes no
mention of some third object called C. So, because the action force is
between Earth and the book, we know that the corresponding reaction force must also
be between those two objects. Therefore, we know that option (D)
is the correct answer, so let’s fill in the blanks.
Thus, we have “Imagine a book rests
on a table. If the force due to Earth on the
book is the action force, then the reaction force acts on Earth by the book.”
