Did I find an error in webassign?

[FCguy]

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

 

[SammyS]

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

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WebAssign is right (at least in this case).

 

[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%. :)

 

[FCguy]

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%. :)

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.

 

[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

 

[FCguy]

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

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.

 

[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.

 

[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.

 

[tony873004]

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.

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.

 

[FCguy]

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.

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.

 

[tony873004]

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