Physics 271: Advanced Honors Physics I
Recitation #3: Problem Solving Checklist for
Newton's Laws Problems
Fall 2017
0. NEVER write only the answer. You need to show where it came from
and why it is right.
APPLIES TO ALL PROBLEMS
1. Check that you have drawn at least one diagram and labelled it
clearly. This might involve copying the diagram from the problem
statement and adding extra labels, or making a diagram from scratch.
Make it pretty large to leave room for labels and make it readable.
Quantities from the statement of the problem should appear in the
diagram.
MOST NEWTON'S LAW PROBLEMS INCLUDE A DIAGRAM SHOWING THE SYSTEM
LABELLED WITH GIVEN QUANTITIES (MASSES, FORCES, ACCELERATIONS,
VELOCITIES). YOU NEED TO ADD LABELS DEFINING COORDINATES FOR EACH
OBJECT IN THE SYSTEM. THEN DRAW FORCE DIAGRAMS FOR EACH OBJECT IN
THE SYSTEM.
2. Check that the meanings of any symbols you have introduced
yourself are clear. First, if the symbol is a label in the diagram,
make sure the diagram makes its meaning clear. If not, or if it is
not in the diagram, write a short phrase to explain what the symbol
represents.
KK OFTEN DOES NOT GIVE NOTATION FOR THE DESIRED QUANTITY SO YOU HAVE
TO CHOOSE IT FOR YOURSELF. TRY TO PICK SOMETHING NATURAL. FOR
EXAMPLE, IN PROBLEM 3.1 THEY ASK FOR "THE INITIAL ACCELERATION OF
EACH MASS." IF YOU HAVE CHOSEN x AS THE COORDINATE OF THE MASS ON
THE FLOOR AND y AS THE COORDINATE OF THE MASS AGAINST THE WALL, THEN
ONE NATURAL CHOICE WOULD BE x double dot sub 0 AND y double dot sub
0. IN THE SOLUTION, YOU WOULD WRITE "The initial acceleration of the
mass on the floor is x double dot sub 0 and the initial acceleration
of the mass against the wall is y double dot sub 0.
IN THIS PROBLEM (KK 3.1) YOU ALSO HAVE TO INTRODUCE NOTATION FOR THE
FORCE EXERTED BY THE POLE ON EACH MASS. ONE NATURAL CHOICE WOULD BE
vector F sub floor AND vector F sub wall.
3. Check that all vector quantities are written so that their vector
character is clear. In typesetting, boldface indicates a vector
quantity. In a diagram, if you draw an arrow to indicate the
direction and write the magnitude next to the arrow (this is how we
draw force diagrams). In handwriting, put an arrow over the quantity
to show it is a vector. Velocity is a vector, so write v with an
arrow over it. The x component of the velocity is not a vector, so
do not put an arrow over it. In a sum, you cannot add a vector to a
number. In an equation, you cannot have a vector on one side and a
number on the other side.
4. Check that each statement clearly follows from the previous
statement. (This is in the same spirit as Checklist item #0 - you
can't state things without showing where they came from).
**In a Newton's law problem, this flow is as follows. Draw the
diagram of the system and label the coordinates. Write the
constraint equations, if any. Draw the force diagram for each
object. Apply Newton's 2nd law to each object and write the
resulting list of equations. Write any additional equations (for
example, the relation of the friction force to the normal force). Go
over the system of equations to list out the unknowns you want to
solve for and the unknowns you can eliminate. Count the total number
of unknowns to check that you have enough equations, then solve for
the desired quantities.
5. Check that you have stated the answer at the end of the solution.
The answer should be an equation in the form
desired quantity = expression in terms of given quantities
You might want to save writing by just writing the right hand side,
but it is best practice to write the whole thing (this acts as a
check that your expression is indeed for the quantity the problem
asked for).
To make it clear that it is the answer, PUT A BOX AROUND IT.
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Rabe.