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If mass is constant, will an astronaut weigh the same on Moon ?

  1. Oct 15, 2008 #1
    Let say a weighing machine is brought to the Moon. If an astronaut weighs 70 kg on Earth, will the machine shows 70 kg on Moon? By right it should because mass is always constant?
    But I am not sure.
    Just like if a man is 60 kg at the Equator, will the same weighing machine shows 60 kg if he weighs himself in London?
  2. jcsd
  3. Oct 15, 2008 #2


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    Irrespective of where the astronaut is, her mass is going to be the same. The weight is a measure of how much gravitational force is acting on her (because, the weighing machine actually measures the force she exerts on it due to gravity, and is calibrated to display your mass on earth, since we know F=mg). So, in empty space, the astronaut will still have the same mass. But now, since the gravitational field is very low, her weight as registered by the weighing machine will be much less.

    See this website for a detailed explanation
  4. Oct 15, 2008 #3

    D H

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    The answer depends on what kind of "weighing machine" you use. Suppose you use one of those fancy scales you see in a doctor's office. This scale will register a "weight" of 60 kg in North Pole, at the top of Mount Chimborazo, or on the surface of the Moon. Suppose, on the other hand, you use a typical bathroom scale. This scale will register a bit more than 60 kg at the North Pole, a bit smaller than 60 kg at the top of Mount Chimborazo, and about 10 kg on the surface of the Moon.

    The difference: A doctor's scale measures mass, while a bathroom scale measures "weight". To most people (and legally), weight is just a synonym for mass. To physical scientists, weight is a completely different beast. To them weight is either (a) the force exerted on an object by gravitational attraction, or (b) all of the forces exerted on an object except for gravity. Airplane designers and most introductory physics classes use definition (a) almost exclusively. Advanced physics classes use definition (b). Your bathroom scale: definition (b).
    Last edited: Oct 15, 2008
  5. Oct 15, 2008 #4


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    Mass would not change but weight would. "Weight" is the force with which gravity pulls you down on the surface or scale. I suspect part of your confusion is based on using "70 kg" as a measure of "weight". On the earth that force is mg where m is your mass and "g" anywhere on earth does not vary very much from 9.81 m/s2: weight is proportional to mass on the surface of the earth so we can get away with saying "weight" is 70 kg. Since kg is not a unit of force but of mass, what that really means is that the weight is (70)(-9.81)= 686.7 Joules. If you move to a different part of the earth, neither mass nor weight changes measurably.

    If you move to the moon, your mass stays the same (there is a "law" of conservation of mass) but your weight changes.
  6. Oct 15, 2008 #5
    Mass would be equal.
    Problem is how to measure it - you usually measure mass through forces and the force does change. If you compare the force of an unknown body and something with known mass, scales could show correct mass, be it on the moon or jupiter. One simple spring wont work tho.
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