Math Is Fun Forum

mikau

Member

Registered: 2005-08-22

Posts: 1,504

physics progam wrong?

grrr...

in my physics course we have to do weekly homework assignments through a special online program. The program gives questions from the book with a few of the values changed, and you have to enter the answers in the provided textfields.

You get as many tries as you want, and it tells you when you've got the right answer. But this one problem appears to not accept the correct answer:

A 0.28 kg particle moves in an xy plane according to x(t) = - 15 + 3 t - 5 t^3 and y(t) = 26 + 7 t - 9 t^2, with x and y in meters and t in seconds. At t = 0.7 s, what are (a) the magnitude and (b) the angle (relative to the positive direction of the x axis) of the net force on the particle, and (c) what is the angle of the particle's direction of travel?

okay, this is an easy problem but for some reason its not working out. (a) worked fine, but for B it wouldn't accept my answer as correct. I eventually discovered it accepts 139.4 as the correct answer (which i'm pretty certain is absolutely wrong). For (c) I'm certain the answer is -127.84° (or 232.16°) but it says incorrect.

Its also worth mentioning that i did the books version of the problem and i got all three answers correct easy. 

Should i email my professor or did I make a mistake somewhere.

Last edited by mikau (2008-02-08 13:56:30)

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A logarithm is just a misspelled algorithm.

mikau

Member

Registered: 2005-08-22

Posts: 1,504

Re: physics progam wrong?

well I just discovered that it accepts the angle of the position vector at t = 0.7 as the correct answer. Thats not the direction of travel, is it?

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A logarithm is just a misspelled algorithm.

LuisRodg

Real Member

Registered: 2007-10-23

Posts: 322

Re: physics progam wrong?

My physics class is the same. Which one do you use? We use Mastering Physics.com in which we do our weekly homework. Up to now I havent found a problem that is bugged but it is bound to happen I guess.

mikau

Member

Registered: 2005-08-22

Posts: 1,504

Re: physics progam wrong?

We use WileyPlus. So far its been pretty good. This is the first thing i've found so far.

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A logarithm is just a misspelled algorithm.

luca-deltodesco

Member

Registered: 2006-05-05

Posts: 1,470

Re: physics progam wrong?

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(angle taken clockwise from x-axis)

were you including acceleration due to gravity in this part of the question? That would be incorrect, since you are given an exact equation for the trajectory, of whos second derivitive is already going to include acceleration due to gravity, or it wouldn't be an equation of the trajectory.
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(again, taken clockwise from x-axis)

Last edited by luca-deltodesco (2008-02-08 23:15:12)

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mikau

Member

Registered: 2005-08-22

Posts: 1,504

Re: physics progam wrong?

yeah, luca. Those are the same answers i got initially.  It would seem the program is in fact wrong.

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A logarithm is just a misspelled algorithm.