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A mnemonic for using Newton’s third law correctly
Mnemonics are tricks for memorizing things. For instance, the musical
notes that lie between the lines on the treble clef spell the word FACE,
which is easy to remember. Many people use the mnemonic
“SOHCAHTOA” to remember the definitions of the sine, cosine, and
tangent in trigonometry. I have my own modest offering, POFOSTITO,
which I hope will make it into the mnemonics hall of fame. It’s a way to
avoid some of the most common problems with applying Newton’s third
law correctly:
P
air of
O
pposite
F
orces
O
f the
S
ame
T
ype
I
nvolving
T
wo
O
bjects
Example
Question: A book is lying on a table. What force is the Newton’s-
third-law partner of the earth’s gravitational force on the book.
Answer: Newton’s third law works like “B on A, A on B,” so the
partner must be the book’s gravitational force pulling upward on
the planet earth. Yes, there is such a force! No, it does not cause
the earth to do anything noticeable.
Incorrect answer: The table’s upward force on the book is the
Newton’s-third-law partner of the earth’s gravitational force on
the book.
# This answer violates two out of three of the commandments of
POFOSTITO. The forces are not of the same type, because the
table’s upward force on the book is not gravitational. Also, three
objects are involved instead of two: the book, the table, and the
planet earth.
Example
Question: A person is pushing a box up a hill. What force is
related by Newton’s third law to the person’s force on the box.
Answer: The box’s force on the person.
Incorrect answer: The person’s force on the box is opposed by
friction, and also by gravity.
# This answer fails all three parts of the POFOSTITO test, the
most obvious of which is that three forces are referred to instead
of a pair.
Chapter 5Analysis of Forces