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Conservation of Momentum, Everygy, Etc

  1. Apr 11, 2009 #1
    Here's some homework True/False questions I got for homework, need some help making sure my answers are right.

    1)In a collision between a laight hydrogen molecule and a heavy water molecule, the momentum lost by one molecule is exactly the same as the momentum gained by the other molecule.

    2)Mechanical energy is conserved whenever momentum is conserved.

    3)Kinetic energy is conserved only in perfectly elastic collisions, but momentum is conserved in all collisions.

    4)Two cars collide, lock bumpers and eventually slide to a stop due to friction with the roadway. During this entire process, mechanical energy and momentum are both conserved.

    5)If the dot product of two nonzero vectors is zero, those vectors must be perpendicular to each other.

    6)When a moving object hits a stationary object and causes it to move, some of the moving object's kinetic energy is transformed into momentum in the object that was at rest.
  2. jcsd
  3. Apr 11, 2009 #2


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    So what are your answers?
  4. Apr 12, 2009 #3

    But if you're gonna be that much of a prick about it, just don't worry about it.
  5. Apr 12, 2009 #4
    I'll just write out the logic/questions I'd have for each of them to suggest the answers I'd pick.

    1) Linear momentum is conserved

    2) First, what is your teaching meaning by "mechanical energy"? Momentum conservation doesn't imply any kind of energy conservation, it exists independently of energy conservation (each conservation law comes from a particular symmetry of the system), but I'd just be careful to make sure that this "mechanical energy" isn't being defined from momentum in some circular manner.

    3) Well, yeah... that's basically just a statement about the type of system you are talking about.

    4) I'm not a physicist, but I honestly don't know what "mechanical energy" is referring to in this context. This is an inelastic collision though...

    5) Try coming up with some cases here...

    6) I'm not honestly sure if this is just really sloppy terminology, or an elaborate trick question... I'd have to know the level that this question was being asked at to give a serious answer.
  6. Apr 12, 2009 #5
    I agree with your answers to 1 and 2. but for the rest,

    3 is true based on the principles of elastic collisions and conservation of momentum
    4 is false if mechanical energy is referring to kinetic energy since energy is lost
    5 is true - its just a basic principle of vectors
    6 im not too sure about- But I want to say false because of the way that it's worded.

    I'm pretty sure of these answer, although I cant say that I'm 100% sure because of the weird way that theyre worded.
  7. Apr 12, 2009 #6
    Yeah, I agree here. The wording is incredibly poor in the original questions, but these should be the answers, within the assumption that this is high school level physics?
  8. Apr 12, 2009 #7


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    Nice attitude. :rolleyes: Grow up.

    The rules (that you agreed to when you signed up with this forum, by the way) say you must show your work before you can expect help. You did not do that, so I'm going to tell you to do it. Your post is typical of those you would expect from some lazy student looking for free answers.

    The homework template is there for a reason, please use it.
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