Is there any type of electronic scale that can measure mass rather than weight? Perhaps one with a strain gauge?
And why do we Earthlings talk about weight but measure it in Kg, so we are really talking about mass?
I suppose you could put the mass on wheels and accelerate it with a rope attached to a spring with a known spring constant and see how far the spring extends, but it’s a lot easier in most realistic Earth-bound circumstances just to measure the weight of the object and convert to mass.
Because on the surface of the Earth, it is a very good approximation to say that weight has a one-to-one correspondence with mass with a constant conversion factor of 9.8 Newtons/Kilogram
Weight is a measurement of mass within a gravity field. On the moon a kilogram of flour would be the same size but weigh only 1/6th. In space the same kilogram of flour would weigh close to nothing but would have inertia meaning you’d need the same amount of mass (or equivalent energy) to move it.
A mass-measuring device that didn’t rely on direct comparison with a known mass (e.g. a balance) could spin the mass at a known speed at a known radius and measure the resultant force, then calculate the mass from that. If this is done within a gravitational field, one would need the plane of rotation to be horizontal.
Strictly speaking, the kilogram is indeed a unit of mass, but of course most folks use it as shorthand for kilogram force, the amount of force corresponding to one kilogram accelerated at 9.81 m/s[sup]2[/sup]. Conveniently, the units have the same numerical magnitude here on earth: a person who weighs 75 kg-f will have a mass of 75 kg, just as a person who weighs 165 lbf will have a mass of 165 lbm.
Dunno about “electronic,” but a simple scale (what you want to “weigh” on one scale, pre-measured “weights” on the other) is comparing masses rather than weights, nomenclature aside!
Because weight is easy to measure but mass not so much. Also for casual measurements you can assume that g=9,8 and derive the mass from the measured weight.
Because for the majority of human history, we’ve been stuck on the surface of this rock, where the distinction is (for most practical purposes) moot.
As Noone Special points out, any balance is a mass measuring device, not a weighing device. A strain gauge is just another spring, albeit one with a built in readout, so it is a force measuring device.
You could use a torsion balance and measure the gravitational attraction (force again) between your test mass and some other known masses. It isn’t all that accurate, and takes quite a while. But knowing the gravitational constant you can get a measure. This is how the gravitation constant was first measured. Of course the bigger the known mass the better, and a planet makes a good mass, but that gets us back to a spring gauge.
About the only way left is to measure the inertial mass. So you need to accelerate it with a known force and measure what happens. One way of doing this in a useful manner is to place the test mass on a spring and measure the period of vibration. This depends upon the spring constant and the inertial mass. This is how some micro-machined test equipment works to measure very very small masses.
Ideally you wouldn’t want to be spinning your test object or flinging it around. The device could have a regular spring-loaded platform for the test object and, internally, have a reference mass on another spring scale to calculate the local gravity/acceleration.
You could average the force over a whole number of rotations and then the orientation of the plane of rotation to any extant gravitational field wouldn’t matter. A bigger problem would be determining the radius, as it is the radius to center of mass of the object that matters, and that may be fussy to determine. You’d probably need to make the radius inconveniently larger than the object so that it wouldn’t much matter how the mass was distributed through the object.
You could do it with a barometer.
Take the barometer to someone who knows the mass of the object in question, and say, “dear sir, here I have a very fine barometer. If you will tell me the mass of this object, I will give you this barometer.”
No, no, no…! You use a barometer to measure altitude, not mass! *
- I’ve got to say, “height and not weight” sounds a lot catchier!
If you did this in a vertical plane within a gravitational field, the average force would be the sum of the weight of the object and the force due to the machine flinging it around. If you know the magnitude of the gravitational field and the orientation of the plane of rotation, then you can account for it. If you don’t know local gravity - let’s say you’ve just set down on LV426 and you want to determine the mass of this egg-shaped thingie - then your best bet is to assure that the plane of rotation is horizontal.
You could have the machine measure the force while swinging at two different radii. Knowing that centripetal acceleration is directly proportional to swing radius, you could produce a system of two equations and two unknowns:
F[sub]1[/sub] = m * w[sup]2[/sup] * (R[sub]1[/sub] + x)
F[sub]2[/sub] = m * w[sup]2[/sup] * (R[sub]2[/sub] + x)
Known:
F[sub]1[/sub], force when swinging with machine set to R[sub]1[/sub]
F[sub]2[/sub], force when swinging with the machine set to R[sub]2[/sub]
w, angular velocity (radians/s)
R[sub]1[/sub], first swing radius measured from center of rotation to bottom of arm’s bucket
R[sub]2[/sub] second swing radius measured from center of rotation to bottom of arm’s bucket
Unknown:
m, mass of object in machine’s bucket
x, distance from object’s CoM to bottom of machine’s bucket (e.g. for a spherical object of diameter 1 inch, x = 0.5)
Now you can solve for m and x, regardless of the shape or size of the object.
J-Cubed: So where did I originally read the barometer thing, but about how to measure the height of a building?
So the old fashioned doctors office scale measured mass. My bathroom scale at home measures weight, but gives the result in kg. rather than the correct newtons.
Or, you bring along a simple old-fashioned balance and masses from, say 1mg, up in multiple of 2 up to, say, 2^20 mg (a bit over 1 KG)
This will allow you to measure mass of anything up to ~2 Kg to an accuracy of 1-2 mg. Payload required, maybe 3 KG including the balance 
The beauty of this is that you can fashion a new, larger (up to 2KG) known mass by using one of the objects you pick up on the surface and measuring it against your “weights” set. Lather rinse and repeat for ever larger masses. Well, up to the load limit of your balance, anyway.
That’s great, but the OP asked for an electronic device that can measure mass.
I’m wondering if the OP wasn’t thinking, “well, clearly we can’t just measure mass mechanically, but maybe we’ve developed something electronic to do it.”
We’d need the OP’s input on this however.
Or, attach a few LEDs to it, maybe a laser device to tell us when the scale is really balanced, and call it electronic… :p:D (OK, J/K there.)
It’s an old, old, joke. But I first heard it in the 80s sitcom “Head of the Class”.
It was passed out in high school science classes in the 70s, on ditto paper. But according to Snopes, it’s older than that.
Hmm.. I always thought you were supposed to take the barometer to someone who knows the mass of the object in question, and say, “dear sir, here I have a very fine barometer. If you don’t tell me the mass of this object, I will hit you over the head with this barometer.”
I mean, who really wants a barometer? But nobody likes being hit over the head with hard pointy things.