## MAE 325 Prelim 1 grading issues, Fall 1999

## 12 point bonus added to all student grades.

Add this to the score you seen on your graded exam.

**If you would like your grading reconsiderred:**

**1)Do not mark even the smallest thing in your exam book,**

2) Make sure you understand thoroughly how to do the problem,

3) ** Read the solutions,**

4) Read the text below,

5) Write a description of why you would like your exam
regraded and give it,

with your exam, to your TA to give to the grader.

**======================================================================**

## Problem
1 - Jie Bie

## If doing
part a:

## Clear and
complete FBDs, LMB and AMB equations --- >

Eight
equations for nine unkowns -
15 points

## The correct
answer is worth 5 points.

## Finding
the correct geometric relation is worth 8 points.

## Knowing
how to use the two-force component principle - 7 points

## If doing
part b:

## Correct
answer - 5 points

## Correct
formula for velocity - 10 points

## Find correct
geometric configurations - 20 points

## If you
did both problems: I chose the one that gave you the most credit and allocated
bonus points according to how well you did the other one.

**======================================================================
**

**Biggest
mistakes on the second problem: -Nathon Barton**

** (*)
application of the "3-force body" rule in a completely incorrect way, sometimes
moving the location of the rope so that it matched the case in the homework;
looks like some people have learned the trick without learning where it comes
from, when it is applicable, and what the special cases look like; the 3-force
boy rule comes from A.M.B. and in this problem, the easiest route was to apply
A.M.B. about the point of contact between the wheel and shaft**

** (*)
use of phi (as in mu = tan(phi)) to indicate some angle in the setup of the
geometry without any reasoning for the choice or proof of why it should be so;
some students got lucky and picked the correct angle, others did not; indicates
a rather fundamental misunderstanding of how to set up problems, especially
those involving some aspect of the geometry/configuration as part of the solution;
regardless of whether or not the student got lucky and picked the right angles
as being phi, points were deducted if the determination of the angle was not
in some way written in the answer to the problem **

**Other
notes on grading of problem 2: **

**Most
of the grade was assigned based on part a -- correct completion of part b was
worth 25 points if part a was not done correctly, but correct completion of
part a without any of part b was worth up to 30 points. Then again, those who
got part a completely correct were rather likely to get b as well. **

**Some
points were awarded for correct FBD, AMB, and LMB, even if the equations were
based on erroneous assumptions about the geometry of the problem. Further points
were awarded for recognition that the force at the wheel/shaft contact acts
vertically, or if some equivalent solution of the location of the point of contact
was found.**

** An honest
attempt was made to check the answers for parts of the solution that were correct,
even if only correct given errors earlier in the problem. **

**======================================================================
**

**Mistakes
on third problem: - John Durkot**

**By far
the most common mistake was in writing the radius of C wrt D. I saw just about
every possible combination of sine and cosine of theta3 (or theta3 - 90, theta3-180,
180-theta3, and 90-theta3). I did not take off many points for this mistake
and tried to follow the problem through with the incorrect radius. If you got
everything correct the rest of the way no other points were decucted. **

**Another
mistake that I found was mistaking M2 for theta3. M is the moment and theta
, or omega, is the angular velocity. Again, if your method was sound but you
made this mistake I only took off the points for the initial mistake. **

**Very
few people attempted to do part A first. Most people did part B and then used
the power balance to do part A. Half credit was given if the problem was set
up correctly (vD=vD or something along that line). **

**A note
on MATLAB code. If you say you would use MATLAB you have to include code, not
just say "use solve command in MATLAB".**