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Homework Help: Inelastic collision of railcar

  1. Mar 10, 2008 #1
    Ok, the question is the following:

    "In a railroad switchyard, a 56 ton freight car is sent at 7.0 mi/h toward a 31 ton car that is moving in the same direction at 26 mi/h."

    - what is the speed of the pair after they couple together?
    -what fraction of the initial kinetic energy was lost in the collision?

    --> My questions is, to find the momentum, do I need to convert the units to kg? I have seen it in examples where the units are not converted. Does it matter?

    --> Also, the fraction of energy lost is expressed as lost energy/initial energy , is that correct?

  2. jcsd
  3. Mar 10, 2008 #2
    You don't have to convert to SI, but it makes everything so much easier and nicer. I learned physics (and I'm assuming most people are the same... and hoping) all in SI, so to me it just feels nicer :P It wouldn't be difficult; just look up a conversion table on google, or something.

    For the second part, I would say that the question is asking what the ratio of the initial kinetic energy to the final kinetic energy.

    In other words, initial kinetic energy/final kinetic energy.

    Hope that helps!
  4. Mar 10, 2008 #3
    Ok, I see.

    My point was, if I do the calculations with tons, the answer for momentum will be in mi/h, right? So, that is really no point in converting to kg, it is easier just to look at final answer and convert it to SI (m/s) then. Is that a correct way of looking at it?
  5. Mar 10, 2008 #4


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    I agree 100%. :smile:

    Examiners will take marks off if you create extra work for yourself!

    Actually … don't even bother to convert it to m/s at the end unless you think the examiners want you to.
  6. Mar 10, 2008 #5
    This is one of the reasons why I dislike the English system of units; it can lower a students confidence in solving problems while they are trying to learn the concepts.

    As you probably know, kg is mass and not weight. In a pound is the English systems unit of force (where as it is a Newton in SI). For mass, the English system has units known as Slugs. So if you have a 10 lb weight, then it has a mass of about 0.31 Slugs [(10lb/32.2ft/s^2)=0.31 Slugs]
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