Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Which has more mass?

  1. Feb 25, 2004 #1
    100 kg of goose feathers or 100 kg of mass and why[?]
  2. jcsd
  3. Feb 25, 2004 #2
    Kilograms are a measure of mass.

  4. Feb 27, 2004 #3
    Obviously, 100 kg of anything is going to have the same mass as 100kg of anything.
  5. Feb 27, 2004 #4


    User Avatar

    Staff: Mentor

    I asked this in another thread: How much does a 150lb man weigh on the moon?
  6. Feb 27, 2004 #5
    Earth pounds or Moon pounds?

  7. Feb 28, 2004 #6
    Mass and wieght are two entirely different things.

    A 150 lb man would weight about 25 lb on the moon. But he would still mass 150 lb.
  8. Feb 28, 2004 #7
    Weight is a force and should be measured in Newtons, not in lb.
    The SI unit for mass is Kg
  9. Feb 28, 2004 #8
    The SI unit for weight may be the Newton, but it's perfectly legit to use the pound (lb) for weight. If someone uses the pound to describe an object's weight and not the Newton, so be it, there's nothing objectable to that. You can very easily switch between pounds and Newtons with a simple conversion, anyways.
  10. Feb 28, 2004 #9
    You can't use the pound for both mass and weight - they are completely different quantities!

    The SI unit for Mass is Kg, the Imperial unit is the pound.

    The SI unit for weight is the Newton, the Imperial unit is the pound-force, not the pound.
  11. Feb 28, 2004 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It entirely depends on what Unit system you are using.

    In the absolute FPS system, the unit of force is the poundal and the unit of mass is the pound.

    In the gravitational FPS system, the unit of force is the pound and the unit of mass is the slug.

    In the American Engineering Standard, the pound is used for both mass and force. (sometimes designated as lbm and lbf , but many times you are just supposed to know what it is being used for by context.)
  12. Feb 28, 2004 #11


    User Avatar

    Staff: Mentor

    That's if he weighed 150lb on earth....who said anything about what he weighs on earth? :wink:
    In the US we do - layman and engineers alike. Heck, if you make your bathroom scale read in kg, its using kg as weight!

    For more on lb vs kg, etc, see THIS thread in the General Engineering forum.

    [/shameless plug]
  13. Feb 29, 2004 #12
    No wonder NASA sent a probe to Mars that missed completely as someone had used the wrong units for thrust calculations! :smile:
  14. Feb 29, 2004 #13


    User Avatar
    Science Advisor
    Homework Helper

    Actually, there are situations where pounds (force) and pounds (mass) are both used. For example, calculating specific impulse.

    Specific impulse is a unit that describes the amount of thrust that is achieved per mass of fuel consumed per second.

    In SI the units of specific thrus are:
    Newtons / (Killograms/Second) = Meters Per Second.

    (This is accurate since specific impulse is essentially determined by the relative velocity of the rocket exhaust.)

    In standard the units of specific thrust are:
    (Pounds (force))/(Pounds (mass)/Second)= seconds
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook