Posted May 30, 2002 Common errors made on Quiz 2
For 3iii, The change in kinetic energy should have been _negative_
indicating that
KE was LOST during the process (and dumped into the "thermal
energy system").
Sometimes a sign error is just a minor algebra problem, but in this case,
important physics is in there and more points were taken off than for
"just an algebra error".
The change in KE is given by KE_final-KE_initial. The order of
"initial" and "final" is important.
The "initial" and "final" are "states of the system". Some people took
the difference in KE between individual particles, which is incorrect.
While conserving momentum for problem 3ii, the signs on the momenta are
important since momentum is a vector. If the m1v1 and m2v2 had the
same
sign with no signal indicating special notation, points were taken
off.
Some people demonstrated that they knew the "real" sign later in the
problem when numbers were introduced. However, for 3ii, the answer should
have been self contained in terms of the variables given.
Posted May 15, 2002 Common errors made on the midterm
General error in solving Newton's Laws: Mistakenly combined the sum of
forces in the x and
y directions into one algebraic equation (e.g. sumF=F_x+F_y=ma)
Remedy: Treat the x and y directions independently (sumF_x=ma_x,
sumF_y=ma_y)[note: I will use _ to indicate a subscript]
Problem 1 and 3: Included friction in the problem unnecessarily.
Remedy: Only include friction if it says to.
Problem 2: Solved for t at maximum height rather than total time in the
air.
Remedy: Follow the instructions in the problem and know exactly what you
want to solve for.
Problem 2: Included velocities and accelerations in the force diagram.
Remedy: Only include _forces_ in the force diagram.
Problem 3: added reaction forces to find the rope tension (e.g.
T=F_(rope on 1)+F_(rope on 2))
Remedy: I commend the effort to write it in "A on B" format. However, if
you use the notation "rope on 1" and "rope on 2" you must then apply
Newton's Law to the rope itself and get the tension that way. For
example, if the rope is massless, then sumF_(on rope)=0 (always), so F_(1
on rope)=-F_(2 on rope) (notice that we are looking at the forces
on the rope). The tension is then DEFINED (for a massless rope) as
|T|=|F_(1 on rope)|=|F_(2 on rope)| (note the absolute values).
In other words, the tension is just equal to ONE of the reaction
forces, not their sum.
Problem 4: Wrote mv^2/R+mu*m*g=ma rather than mu*m*g=mv^2/R.
Remedy: Remember that for a particle in circular motion the radial
acceleraion is v^2/R. There is no extra force called mv^2/R. The term
mv^2/R IS the "ma" part of sumF_radial=ma_radial.
Posted May 13, 2002
Look for "score" at the far right of the following link. Also see the bottom of the following link for more information
about grading etc: Midterm 1 Grades by last 4 number of Student ID
Midterm categories
Posted April 25, 2002 Midterm
Information
Posted April 25
Quiz 1 stats:
Total points possible: 37
Class average: 31.9 (86%)
Class median: 33 (the same number of people received scores above 33 as
below)
Rough grade breakdown (subject to minor changes):
A>33
B>29
C>25
Problem 1:
Possible points: 6
Average: 4.8
Problem 2:
Possible points: 6
Average: 5.0
Problem 3:
Possible points: 5
Average: 4.3
Problem 4:
Possible points: 20
Average i: 9.3
Average ii: 8.4
Average for problem: 17.7
Categories for problem 4 parts i and ii (each part received one of the
following categories). Each part was worth 10 points.
Q=10 Perfect
R=8.5 Almost Perfect. Minor algebra errors. Sign error.
S=7.5 Some meaningful result but incorrect answer. Major algebra errors.
Minor formulatic error.
T=6.5 Good effort but incorrect answer. Major algebra formulaic, or
conceptual errors.
U=0 Blank or no meaningful information.
Posted April 11, 2002
Quiz 1 will be on Monday April 15, 2002. It will last about 20 minutes
and be given in lecture.
The quiz itself will be designed to test basic knowledge. If you have
attended lecture and discussion, understood the material presented,
and
made an effort to do the homework, you should do very well.
Treat the discussion problems as practice quiz material. No calculators
will be required.
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