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Himanshu Ranjan Dwivedi

weighing machine

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7 hours ago, Henry Kurniadi said:

Mass.

Because the machines are (by law) annually calibrated by standard mass that was kept by Government Metrology Bureau.

There are no such thing as standard weight.

 alright

but you know if we stand on the weighing machine, we affected with the gravitational force and gravitational force pulls us at that time. so don't you think it will be our mass as well as gravitational force? which will lead to our weight.

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For starters, we might consider just what sort of "weighing machine" you have in mind. There are basically two types:

1) the beam balance, where the position of a sliding block is adjusted to the point where the moments on the beam are in equilibrium;

2) the force cell (usually a strain gage device) that is actually a spring scale with a very stiff spring.

Either of these can be calibrated in either force or mass units, but they operate on different principles (or do they?), so do they measure the same quantity?

DrD

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The original question posed was this: "what does a weighing machine measure our weight or mass?"

To measure anything simply means to compare an unknown to a standard of the same type. Thus to measure a length of string, you compare the unknown length to the length of a ruler and discover that it goes, for example, 5 and 1/4 times. Thus the unknown is 5.25*(ruler length). OK, I'm sure everybody knows that part.

To compare to items, they must be of the same type. Thus you can compare one length to another length, but you cannot compare a length to a time interval. You can compare one force with another, but you cannot compare a force with a displacement.

One way to look at this question is to ask a second question: Do we have any means to compare one mass with another? Mass"what does a weighing machine measure our weight or mass?" is the quantity of matter, the amount of "stuff" in the material. That "stuff" includes electrons, protons, and neutrons, each of which contribute a little bit of "stuff." The obvious answer is no, we cannot compare masses. We cannot count the number of protons, the number of neutrons, and the number of electrons in on body to compare with those in another body.

When we talk about weighing something, we are determining its weight, which is a force. It is important that we keep in mind that weight is a force, the force of gravity on the body.

As I mentioned in the earlier comment on this topic, there are basically two types of scales, the common term for a weighing machine. There is the beam balance type, where a known weight is moved along a beam in order to balance the beam and create an equilibrium situation. There is also the spring scale in which an elastic member is deformed, and the amount of deformation is an indication of the weight applied.

When we weigh something, we are comparing forces and the torques developed by those forces. We say the forces are equal when we have established an equilibrium condition, and we can do this because this requires that the sum of forces be zero.

In the beam type balance, we put an unknown weight in the pan, and then adjust the known weight so that the upward force through the mechanism is equal to the weight; this is the required equilibrium condition. All of this is a matter of comparing forces.

In the spring type scale (including the modern electronic read-out strain gage based scales), we are deforming a spring in order to support the weight of the test object. The measure of that deformation (a displacement) is taken as the measure of the weight. This is true wither the displacement is many millimeters (as in the common fish scale) or only a few thousandths of a millimeter as in a strain gage scale.

To go one step further, consider the following thought experiment. Imagine a scale (or either type) under a low shed roof. Now, put an empty balloon on the scale and weigh it. It will weigh very little, even if it is a rather large balloon.  Next step, begin to inflate the balloon. For a while, as the balloon swells, you will only see a tiny increase in weight due to the weight of the air enclosed, but keep going. Eventually, the balloon will begin to bear against the shed roof, so that with increased air pressure, the balloon pushes up against the roof and down against the weight pan. After contacting the shed roof, the addition of even a small amount of air to the balloon will show a marked increase in indicated weight.

Does this mean that the last little bit of air you added was much heavier than that previously pumped in? No, the last bit was just like the first part. It means that your scale is no longer indicating weight (gravitational attraction), but is showing the effect of the pressure loading from the internal pressure of the balloon. Add a little bit more air, and you will see a significant increase in indicated weight, but again, this is not truly weight but the force of air pressure.

Even at this advanced stage in the process, the actual mass of the balloon and the air contained are very small, but the indicated weight is quite large. This is because it is no longer showing weight, but the force of the applied air pressure.

The point of the thought experiment is to show that it is downward force that is read by the scale, not mass sitting in the pan. The answer to the original question is, "the scale measures weight (a force), not mass."

DrD

 

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