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What experiments have been done to check that objects weigh the same in different orientations?

To what accuracy has it been checked that the two weights of the same object in the positions below (for example) are the same?

enter image description here

At first sight it's self evident that they should weigh the same, but apparently Isaac Newton wondered about it. In those days the accuracy of any check would be low. Has there been any modern accurate follow up for this?

There is a possible connection to Mach's principle and whether inertia is caused by the amount of compression that occurs in a body when it's accelerated, reference frame for acceleration.

In this case, the mass is compressed by different amounts when resting on the scale, but does that cause any difference in the weight?

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    $\begingroup$ After verification of Newton's law of gravitation, $$F_g=G\frac{mm}{r^2},$$it is evident that the gravitational force only depends on mass and not shape. $\endgroup$
    – Steeven
    Oct 2, 2021 at 11:01
  • $\begingroup$ @ Steeven Experiments using orbits of satellites would test the acceleration is independent of shape/orientation, but what about the weight, i.e. force of gravity and the inertia of the object. Has it been carefully checked that both are independent of orientation? $\endgroup$ Oct 2, 2021 at 11:08
  • $\begingroup$ Since Newton formulated the law of universal gravitation and used it to explain the tides on earth, if he had though about it he would have realized that the weight of an object is not constant. I don't know if he every recorded his thoughts on this topic, $\endgroup$
    – alephzero
    Oct 2, 2021 at 11:59

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In a non-uniform gravitational field the weight of an object obviously does depend on its orientation as well as on its location. So for the sake of argument let’s assume a uniform gravitational field.

Here is a simple thought experiment to show that the weight of a 2x1x1 cuboid in a uniform gravitational field does not depend on which face is uppermost.

First weigh the cuboid when one of its 1x1 faces is uppermost. Now divide it into two 1x1x1 cubes and move the upper cube so that it sits next to the lower cube without changing its orientation. Join the two cubes together again and you now have a 2x1x1 cuboid with a 2x1 face uppermost. Since the orientation of the two cubes was not changed while re-arranging them, the reading on the scales has not changed, so a 2x1x1 cuboid must weigh the same when a 2x1 face is uppermost as when a 1x1 face is uppermost.

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  • $\begingroup$ Yes, that's a good argument for why they are likely to weigh the same. The question does not try to assert that there would be a difference, it's asking about any experimental check. Sometimes things in physics (e.g. time flows equally for all observers) have been considered self evident...but turn out to be different to what was assumed $\endgroup$ Oct 2, 2021 at 11:34
  • $\begingroup$ The answer is clearly "no, they do not weigh the same" unless you can create a perfectly uniform gravitational field. In the thought experiment, you are changing the distance from one small cube to the center of mass of the earth, therefore the cube will have a different weight. Not to mention the fact that the weight in an experiment on earth will be different at different times, depending on the position of the sun and the moon. These effects are small but easy to measure with commercial equipment. $\endgroup$
    – alephzero
    Oct 2, 2021 at 11:52
  • $\begingroup$ @alephzero The original question obviously assumes a uniform gravitational field otherwise, as you say, the answer is trivial. I have amended my post to make this assumption clear. $\endgroup$
    – gandalf61
    Oct 2, 2021 at 11:57
  • $\begingroup$ @gandalf61 I don't like guessing what questions "assume." It could be the OP isn't clear about the difference between weight and mass, for example. The diagrams seem to show a brick on a kitchen scale, which might be a measure of how sophisticated the question is. $\endgroup$
    – alephzero
    Oct 2, 2021 at 12:03
  • $\begingroup$ @ alephzero please treat it as a serious question and would answers say what 'experiments' have been done to check. A uniform gravitational field was assumed, or the earths inverse square field would be taken into account in a good experiment $\endgroup$ Oct 2, 2021 at 12:15
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As has been noted already, the question of performing a critical experiment to verify the hypothesis depends on the existence of a uniform gravitational field.

For practical purposes a uniform gravitational field does not exist, because (unlike electromagnetic fields for example) there is no known way to shield an object from undesirable gravitational effects.

Modern gravimeters can measure to an absolute accuracy of the order of $10^{-9}g$, (where $g$ is gravity at the surface of the earth) so to obtain a null result the gravitational field would have to be uniform to a greater relative precision than one part in $10^{-9}$ or $10^{-10}$.

That is roughly the same change in weight as would be caused by lifting a mass by $1$ mm relative to the surface of the earth.

If that seems unbelievably small, consider that gravimeters are routinely used in archaeology to find buried artefacts. There are also anecdotes where lab experiments using accurate gravimeters produced meaningless results because snow was falling and accumulating on the roof of the lab during the experiment.

For an experiment on earth, the changes in weight caused by tidal effects in the land (typically a few tens of millimeters of vertical movement) and in the changing positions of the sun and moon (which are of course the causes of tides) are all greater than one part in $10^{-9}$.

Attempting to correct for all these effects by calculation is a circular argument, because the theory assumes that the gravitational force (and hence weight) is independent of mass, which is what you are attempting to investigate.

Even if an experiment was attempted in deep space, the results would still be affected by the relative positions of the different parts of the experimental apparatus itself.


As a historical note, since Newton used his theory of universal gravitation to explain tides, he certainly "knew" about these effects in some sense of the word "know". I have no idea whether he wrote anything specifically on the topic you are asking about.

Indeed, every sea-captain in Newton's day also knew about these effects, having observed that pendulum clocks (which in fact measure gravity, given a stable independent time reference such as the rotation of the earth relative to the stars) run at different rates at different latitudes on earth.

It is fair to say that in Newton's day there were competing hypotheses as to why this occurred, one of which was that weight was affected by temperature, since the earth's climate also changes with latitude. The fact that there were no reliable thermometers, or temperature scales, made experiments on this difficult to reproduce, but attempts were made, and which demonstrated that temperature was not a significant factor.

As a later historical note, the first lab-scale experiment to measure the value of Newton's universal gravitational constant (made by Cavendish about 130 years after Newton published his theory) works precisely by using the difference in gravitational attraction between laboratory-sized objects (with mass a few kg) in different geometrical configurations.

There is no particular problem in repeating Cavendish's experiment using equipment in any high-school physics lab, though it requires a lot of patience and is likely to take a few weeks to get the apparatus working correctly, so it is unlikely to be part of a standard lab course! FWIW I did it myself when at high school.

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  • $\begingroup$ Thankyou for the answer. It seems to mention why there would be limitations on how far any difference can be ruled out, that's true for any experiment. The question asked: Have any experiments been done to rule out any difference and to what accuracy? Do you know of any such experiments and the accuracy so far achieved? $\endgroup$ Oct 2, 2021 at 21:21

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