# Did I find an error in webassign?

1. Jun 25, 2015

### FCguy

I think I found an error in a webassign problem. What do you think?

2. Jun 25, 2015

### SammyS

Staff Emeritus
WebAssign is right (at least in this case).

3. Jun 25, 2015

### tony873004

In that term, you should be using 0.05m (12.0 N) sin(60) or cos(30) which gives an answer of -3.85.
-3.77 is within 3% of the correct answer. So now you know your teacher set the tolerance to at least 3%. :)

4. Jun 25, 2015

### FCguy

I should elaborate that I answered with -3.85 and it rejected that answer. Also, our professor informed us that tolerance is 1%. Finally I have a photo of the "master it" help portion in which webassign shows the calculation without the trig.

5. Jun 26, 2015

### tony873004

If it rejected -3.85, then this is a Webassign bug. I teach Physics. I wrote my own "webassign-like" software. I set the tolerance to 1%. It still requires that I do the problems properly. Sometimes I don't. I make the same careless mistakes my students make. In that case, the system accepts wrong answers as right, and marks right answers as wrong. I give EC to the first student that finds a bug (cheaper than hiring an editor!) I imagine the Webassign authors have the same issues. Email them. They'll be happy to get free proofreading.

Is this problem directly from Webassign, or did your teacher create it? Webassign allows instructors to create their own problems. They have to spell out the solutions in Perl programming language. Maybe your teacher made a mistake

6. Jun 26, 2015

### FCguy

I believe these problems are created by webassign, not by my professor. Thanks for the confirm on the answer. I've emailed webassign. Teach on prof! I'm curious as to what level of education you instruct. I never took physics in high school but boy would it have helped now.

7. Jun 26, 2015

### Nathanael

I had the same instinct as Sammy, namely that the 12N force was tangent and the dotted line was a red herring.

The answer depends on where the force is being applied, which is a bit ambiguous from the picture.

8. Jun 26, 2015

### tony873004

I teach high school AP C Mechanics, similar to your first engineering-level Physics class in college.
It comes at you quick if you've never learned it "conceptually", and you're learning it with algebra, trig and calculus the first time you've seen it.
Go to youtube and find as many Paul Hewitt videos as you can. He explains Physics in a way that a 2nd grader could get it. He never uses Calc or trig, and rarely uses any numbers greater than 10. And he's very entertaining.

9. Jun 26, 2015

### tony873004

Actually, you're right! The pull is tangent. 30 degrees doesn't have anything to do with the problem. The angle is 0 from the tangent I fell for the trap. -3.77 is correct. I didn't catch the red herring! Nothing ambiguous here. Like I said a few posts ago, I often make the same mistakes as my students.

10. Jun 26, 2015

### FCguy

I agree with your observation in retrospect but I still believe that this particular red herring was incorporated in such a way that they actually changed the problem. The 12N arm certainly appears to be crossing the circumference of the inner circle. They should have used something like 80 degrees to clearly show they didn't mean for the force to be passing through the circle.

11. Jun 26, 2015

### tony873004

They probably were thinking of a yo-yo where the 12N pulls was pulling on the spindle