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Homework Help: Very Simple Problem, Very Stupid Computer

  1. Feb 4, 2005 #1
    Ok so the question is:

    Code (Text):
    Consider a problem in which a car of mass M is on a road tilted at an angle theta. The normal force:
    And the choices for answers are:

    1. N=M*g
    2. N=M*g*cos(theta)
    3. N=(M*g)/cos(theta)
    4. is found using sum vec{F}=M * a

    from my calculations, i get that N=M*g*cos(theta)...but when i enter that choice it marks my answer wrong. I am getting a B in this class because this stupid f**ing Mastering Physics website...i HATE THIS COURSE....i have basically already taken it but i feel so stupid...i wonder what ppl are feelign like who havn't already gotten a 5 on the AP test for this course...anyways any help will be GREATLY appreciated!!!!!!!!!!!!!
  2. jcsd
  3. Feb 4, 2005 #2
    well im leavin this to you guys...ive waited long enough and am getting tired...ill check back in the morning so if you do know what is going on plz do tell.
  4. Feb 4, 2005 #3


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    Gold Member

    Your answer is right -- I'm not familiar with the way you enter answers into the Mastering Physics website though. Perhaps you're just entering it improperly?

    - Warren
  5. Feb 4, 2005 #4


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    Is it specified how the car moves or it does not move at all? I would choose the fourth answer. For example, if the car has been just dropped of a cliff and still in the process of crash :) or if it goes round a bend and its velocity has a component that is perpendicular to the direction of the tilt, or it is just lifted by a crane, the normal force is different from mgcos(theta).

  6. Feb 4, 2005 #5


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    I agree with ehild. Are there more details to this problem? Is frictional force given? Is it going in a circular motion?

    If no more information is given, then the answer seems to be 4.
  7. Feb 4, 2005 #6
    the car is on the road.
    seeing that there isn't any sin(theta) in the answers, i wouldn't expect any trickery as to what theta is refering to (tilt-wise)
    normal force is just the force perpendicular to the surface and acting on the car, so there is no need for a frictional force to be given
    anyways, i think xerxes answer is correct.

    Edit: Well actually, maybe that crane idea might change the answer a little haha. :biggrin:
  8. Feb 4, 2005 #7


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    I was thinking perhaps it was a circular track at an angle theta, and the car moving on the track at a constant speed. Then we have a centripetal acceleration. So N=mgcostheta is wrong in this case. If friction is negligible then in this case then:
    So N=mg/costheta

    I agree if the car is not running then the answer is mgcostheta. But all we have is the car is on the road. :confused: The car could be doing anything on the road for all we know. 4 seems the most general answer.
  9. Feb 4, 2005 #8
    I am also using mastering physics. It is a POS program. What school and year are you taking physics in?
  10. Feb 4, 2005 #9
    haha,, I had to use mastering physics... it is a POS. Can be very frustrating because even if your answer is right, it might not be submitted in a way in which the program can understand it... I feel for you!
  11. Feb 4, 2005 #10
    Happily, I didn't have that pleasure. Clearly Xerxes' answer is correct if the car is stationary, and as the question says nothing about motion it seems reasonable to assume it is standing still. And the "correct" answer is phrased in such a way as to look like a trap.

    People who create learning systems (books, programs, whatever) with tricky, ambiguous questions like that one should realize that they make the educational process frustrating and unpleasant for many students, and therefore less productive than it should be.
  12. Feb 5, 2005 #11
    stupid freaking homework

    ya...it was multiple choice so no chance of me entereing it in wrong. we arn't even on the part with centripital acceleration (i know how to do it, i already took a course similar to this) ...this is supposed to be chapter 3 homework...CHAPTER 3!!! jesus this is stupid...i asked my professer about it and he was like "uhh....i dunno...maybe go ask one of the other students for help"...HELLO! YOU ARE THE ****ING PROFESSOR...DO SOMETHING OTHER THAN USING PREMADE POWERPOINT SLIDES AND PREMADE HOMEWORK ON THE WEB THAT YOU DON"T EVEN HAVE TO GRADE...jeez...a 4 year old could do his job....gah if i didn't need the credits for my gpa i would test out and go to a higher course
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