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Regarding unlearning, in the sense of cancellation, here are some examples:
*) The textbook says by way of definition that «An object in
mechanical equilibrium is stable....» Unlearning is when you
find out that mechanical equilibrium does /not/ imply stability.
*) The textbook says «Most of the cannonball’s momentum is in
speed; most of the recoiling cannon’s momentum is in mass.»
Unlearning is when you find out that momentum is never "in"
mass or "in" speed ... and even if it were, it is a proverbially
bad idea to compare apples to oranges.
*) The textbook says «The Rule For Toppling. If the center of
gravity of an object is above the area of support, the object
will remain upright.» That statement appears directly underneath
the picture of a tall, narrow, double-decker bus. Unlearning
is when you discover that keeping the center of gravity above
the area of support is absolutely not sufficient to prevent
toppling under foreseeable operating conditions.
*) The textbook says «We say that an object balanced so that
any displacement lowers its center of mass is in unstable
equilibrium.» Unlearning is when you find out that's just
not true.
*) The textbook says «The equation E=mc^2 is more than a formula
for the conversion of rest mass into other kinds of energy, or
vice versa. It states that energy and mass are the SAME THING.»
Unlearning is when you figure out that mass and energy are not
the same thing.