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".