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Homework Help: Simple problem that webassign keeps saying wrong

  1. Jun 14, 2009 #1
    1. The problem statement, all variables and given/known data

    The average speed of a nitrogen molecule in air is about 670 m/s, and its mass is about (4.68 x 10^-26)kg.

    (a) If it takes (3.60 x 10^-13)s for a nitrogen molecule to hit a wall and rebound with the same speed but moving in an opposite direction (assumed to be the negative direction), what is the average acceleration of the molecule during this time interval?
    ____________ m/s^2

    (b) What average force does the molecule exert on the wall?

    2. Relevant equations

    Ok, here is the basic kinematic equation and force equation. I used both of these equations to talk to my professor after class 2 days ago to solve it and he got the same answer. I emailed him today and he said you use the average acceleration kinematic equation to solve it but thats what we did, i dont know if he doesnt remember or there is some secret one that i have never been informed of.

    a)- (final velocity - initial velocity) / (change in time) = average acceleration

    b)- (mass * average acceleration) / (change in time) = average force

    3. The attempt at a solution

    a) (-670-670) / (3.6 x 10^-13) = approximately (-3.722 x 10^15)m/s^2
    Webassign's response: "Answer has wrong sign"
    My next answer: ([positive now]3.722 x 10^15)m/s^2
    Webassign's response: "Answer has wrong sign"
    Advice on tricky wording or problem or something?

    b) [(3.722 x 10^15)*(4.68 x 10^-26)] / (3.6 x 10^-13) = 483.86 N
    Webassign's response: "Answer is off by an order of magnitude"
    My next answer: -483.86 N
    Webassign's response: "Answer has the wrong sign"

    Honestly this makes no sense but if anyone can help or gets a different answer you help would be greatly appreciated
  2. jcsd
  3. Jun 14, 2009 #2


    User Avatar
    Gold Member

    For part a) I don't really see what's wrong either other then I got exactly -3.725 instead of your -3.722 but you noted it was approximate. For part b), you don't divide by the time interval. F = ma, and averages can take the place of instantaneous F, a. Remember to check your dimensions, your computation has units of kg*m/s^3 which is not a unit of Force.
  4. Jun 14, 2009 #3
    Excellent point about b) i cant believe that i missed that one thank you very much. But you are saying that there is no explanation for a being wrong. and the .003 off shouldn't matter because as long as you are within 1% or the correct answer it will give it to me.

    EDIT: Your help has been great can you look at my other problem as well:
    Last edited: Jun 14, 2009
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