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Weight/mass discrepancy: Semantics? I say NO!

  1. Oct 8, 2009 #1
    I feel like I'm in the twilight zone.

    I'm at a new university for the year, cause of financial reasons, and I feel like I walked into a George Orwell story.

    Our physics lab told us to "determine the weight of the block in kilograms."

    It bothered me, since kG don't measure weight, but I decided to assume they meant mass and let it go, but then, the procedure had us doing calculations for torque and friction and acceleration and kinetic energy, etc. all using its mass in kG as the force!

    I couldn't bring myself to do the calculations at all.
    What was I supposed to do, write "The lever feels 10 Kilogram-meters of torque"?
    That's nonsense! I found the weight in Newtons first, and of course, this made my answers differ from the rest of the class' by a factor of acceleration due to gravity.
    My professor counted them wrong!

    I took the lab manual to the head of the department and told him my issues.
    "Why does this lab manual use weight and mass interchangeably?"
    His reply:

    "It's semantics!" and "It's a simplification!"

    Am I crazy? Or is it not semantics at all! It's a lie! It's not a simplification; it's sabotage!

    What can I even do about it? No one I've talked to agrees with me! They all say I'm being "nit-picky"! I've gone as high up as I can go without filing a police report!
  2. jcsd
  3. Oct 8, 2009 #2


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    Science Advisor

    Just because the numerical value differs, doesn't mean that the answer differs. Did you explicitly give your answer with the unit "Newtons"?
  4. Oct 8, 2009 #3
    You can use kg as a unit of force.

    1 KgF = 1Kg (mass) * Gravity.

    I personally dont think it has any place on a physics paper/workbook/textbook as SI units should be used.

    However I do sometimes use KgF in an engineering context, in fact it was to describe a torque. 1 kg.m of torque is far easier to say than 9.81Nm becuase it allows the use of round numbers.
  5. Oct 8, 2009 #4


    Staff: Mentor

    I agree with Chris and A.T.. You can certainly use a non-SI system of units where 1 kgf is the force exerted by gravity on a 1 kg mass. Something similar is done with pounds where it is not always clear from context if a pound is intended to refer to a mass or a force. It can be a pain and a source of confusion, but it is not wrong per se.

    However, this non-SI unit of force would be equal to 9.8 N in the SI system of units, so if you wrote your answer in N then it is correct. However, if you gave your answer without units then I could see the teacher's point. Also, this system of units would either be inconsistent or would require a rather weird unit of time (sqrt(9.8) s) or mass (1/9.8 kg).

    In science it is almost always best to stick with SI units or natural units. Whenever I am reviewing a manuscript that is one criticism that I frequently have for the authors.
  6. Oct 8, 2009 #5
    The lab asked for torque at one point.
    I answered explicitly in Newton*meters.
    Everyone else answered in Kilogram*meters, which is the moment of inertia, not torque.
    I was marked off.
    It even had us use its mass in kG as the force in F=Ma. It read:
    2.2 kG = 2.2 kG * a m/s^2
    That is nonsense! The units don't even make sense! It's like apple=orange!
  7. Oct 8, 2009 #6
    It should be 1 kgF.m of torque, though.

    I'd be fine with such a distinction, had my professor made it.
  8. Oct 8, 2009 #7
    In context you dont need the F bit though. That is nitpicking.
    Also MOI is Kg*m^2

    However if it had units of force and mass in the same eq:
    You are right, that is nonsense as you need the KgF here to distinguish between mass and force. I'd have an issue with this as its then not clear from the context.
  9. Oct 8, 2009 #8


    Staff: Mentor

    Then your answer is correct, just in different units and there is no reason to favor their choice of units over SI units (in fact the opposite is true). IMO, that is like counting off because one student expressed a volume in liters and another used gallons.
    Last edited: Oct 8, 2009
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