A method
for solving
problems. The six rules
(Vectors
will
appear in bold)
1. Draw a
carefully
labeled sketch of the physical setup.
2. If the problem
has been stated in terms of numerical values, you should replace these
with
letters. List what you are using for each value. Fully
solve the
problem algebraically and substitute numbers in at the end of the solution.
3. Attack the
problem from
first principles. Substituting into some formula can't lead to as
much
understanding as an approach from fundamentals. It is also less
likely to
be correct. This means that when you have an acceleration and
need to
find the velocity or position, you must do the integrals. When
you are
working out a mechanics problem involving forces, explicitly list the
forces on
each object in the problem and apply F= m a to each piece of the
problem; let
the algebra glue the pieces back together for you. For some types
of
problems you will start from a form of the work-energy theorem or a
conservation law. Especially don't just look through the book to find a
formula that you
can plug into -- that shows you nothing.
4.
Use
some words to explain what you are doing as you go. Also
states
what assumptions you are making. This would be a good way to
start the
problem, stating what the basis of your solution is to be. It's
not
necessary to write paragraphs, but I require a few sentences scattered
through
the solution.
5.
Analyze the
solution: Check special cases of the parameters entering the
problem; see
what happens if one of the parameters gets very big or very
small. Try to
show that your answer is wrong! This requires that you have done
the
problem algebraically as mentioned in item 2 above. Check
dimensions. When there are numbers, substitute them at the end
and see if
the result is plausible.
6.
Use
vector notion properly. Vectors are not equal to scalars.
Components of vectors are not vectors, they are numbers. The
equal sign
is not just a bit of punctuation. it means exactly and only what
it says,
so you can't set a vector equal to a scalar any more than you can say
that the
population of Miami is two million miles per hour.
The above
rules are
simply the way that people experienced at solving problems always
work.
They take practice, but when you are used to them you will be able to
solve
complex problems far more readily than you would with other, less
structured
approaches. You can, especially with some of the simpler
problems, get a
correct result without such a systematic approach, but a major part of
this
course is learning how to solve problems effectively and efficiently.
Because human nature is
what it
is, to make you pay more attention to the rules as a long-term
investment,
there is going to be a "tax" applied to your score when the rules are
not followed. The tax is going to get stiffer as the semester
moves
along.
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