Weighing a package

When I take my packages to the post office the lady has to move her scale and stack things underneath it to make room for the 72" packages I ship. Couldn’t she just weigh one end of the package and double it. Or weigh each end separately and add the two weights together? My packages are always balanced in the center so no need to weigh each end.

I can see where it might make some difference on how the package sits on the scale so if the platform is say 6" square should the opposite end be held 3" from the end?? When I do this at home it always comes out right on.

You’re introducing another point of force into the equation. Depending upon where exactly you are resting the other end, you can come up with all sorts of crazy variations. With your method at home, move your other support up a half centimeter or down a half centimeter or move it slightly toward the middle of the package. The downward force the package is exerting on the scale is going to change. You could likely do the math and figure it out or be very meticulous with where you place your support and come up with a pretty close approximation OR, you could save yourself a half hour of measuring angles and simply move some crap near the scale.

That may be good fortune, or an artifact of how you pack things, or dumb luck. However, the process you describe, and qualify to the nines, weighing parts of an object, and averaging or summing, is completely wrong. Weights simply aren’t done that way. Its just ignoring basic physics and general knowledge. Its their job to weigh and compute the cost to you. Its the same as for medicine manufacture or architecture – your corner cutting measuring process would be criminal in those cases. No less so for commerce, even government commerce.

The packages in question usually run about 5#. I seldom see a variation of more than an ounce and round off high anyway. It takes her about 5 minutes to get her scale set up. I would happily pay for a weight rounded off slightly high and not have to see her go through all the changes to set up.

You’re assuming that she’s willing to take your word for how well it’s balanced or that she can weigh one end and then the other (which may very well not work, it would be much better to set it on two scales and add them).
For what you’d like her to do, you might as well just tell her how much it weighs or how much you’d like to be charged (as in ‘just pretend it’s 6 pounds’).

Besides, if it’s weighed incorrectly and gets caught later in the delivery chain, the error will fall back to her.

And come to think of it, aren’t their scales tied directly into the register? In other words, I don’t know if they can manually enter a weight.

:dubious: As far as the basic physics goes, it’s spot-on. This is the first thing you learn in an undergraduate statics course: if an object is at rest, the sum of all forces acting on it adds to zero. So if you add up the forces on all the support points, you get the weight of the object.

The devil is in the details, of course, and if you are not perfectly repeatable in how you support both ends of your object, you can easily introduce errors that are unacceptable in the realm of commerce. It’s likely the methods proposed by the OP are prohibited by state commerce laws and/or USPS regulations for exactly this reason. The USPS transaction software is also probably not written to facilitate summing of weights to obtain a single-package total weight, so even if you’re working with a postal clerk who is willing to bend/break the rules, the software won’t let it happen.

Thank you for this, its a better way of describing my objections to the plan, as opposed to my “ignoring basic physics” over-exaggeration.

And thank you as well, for adding yet another personal qualifier, just to this situation, that you wouldn’t apply in any other case. Have you clipped any pennies when buying gasoline to account for the 9/10 rate lately? Have you computed the loss of time and efficiency in the sale moving process, determined the cash value of the tax burden, and petitioned the postal service for your refund?

Hrm. Re-reading, there’s no way I can fix that last paragraph of mine to not come off as mean. I don’t mean to threadshit, I really don’t. However, haven’t enough different people pointed out that we just don’t do these sorts of things you suggest, even if we avoid them just for liability reasons?

I never take offense at a snarky comment or even consider it mean. I figure on some sites we will always be challenged so it is a good idea to try and have our ducks in a row before posting. I very often fall short of this so taking offense would be counter productive for me.

I think under ideal conditions, the summing of forces will work. The issue is a lack of ideal conditions. Of particular interest to me is the relative heights of the points. If the scale is higher than the static rest point, then the weight on the scale will be reduced. There is also the possibility that the contents of the box can shift between weighings and introduce another error.

Ultimately, though, the Postal Worker knows for certain that her method of measuring will be accurate, and isn’t certain of the other options. She isn’t a physicist, and shouldn’t be expected to sort through the possible ways to accurately estimate the package’s weight by using multiple weighings.

I don’t know the physical arrangement of things at the P.O., but they could obviously use two scales and add the readings, that would eliminate any error in the rearrangement process with one scale and a support point.

I actually know you are right but it has become a little bit of a fun challenge to see if I can get her to try it my way. We have gotten to know each other now.

The last time I went I showed her the balance point on the box, I had a level in my pocket to level the box and a 1/2 dowel to set in the middle of the scale that she could rest one end of the box on. She just laughed at me and went about moving everything to fit the box LOL. One of these days I am going to talk her into it.

Why does this matter? I think it just matters that you support the package in exactly the same two positions when you switch the scale and the other support point around.

Is this like weighing yourself by stepping one foot on the bathroom scale, and then stepping the other foot on, and then adding the results?

I was just demonstrating that she would only have to weigh one end.

On a side note just for the sake of math and nothing practical lets suppose that you had a six foot long 4" box and the balance point was 12" toward one side. For some reason you were only allowed to weigh on one end. Basing it on the balance point you could determine at what point to support one end either at the hand or the scale or the angle to hold it at that would make both sides equal. What would be the formula for figuring either of these two ways?

I usually do things like this backward where I physically find the answers and then try to figure out what formula will get me there consistently.

ISTM, if this were any kind of a regular occurrence, they would make the top of the scale higher than the counter instead of lower (where it won’t get bumped into all day).

There is another possibility, it could keep everyone happy but would require training. Instead of lifting the scale to weigh something larger, put something [like an empty box] on the scale, rezero the scale, then set the item to be weighed on top of whatever you put on the scale.

There are two drawbacks to that. Some people, both cashiers and customers may not understand how this works and end up confused and the possibly bigger problem is that if the scale isn’t zeroed again after this is done and everything removed from it, it’ll show all weights going forward incorrectly.

So there is a way, but it’s easy to screw up and could cause confusion. In the end, it’s probably just easier to prop the scale up once or twice a day.

The ceiling is actually high enough she could set the bow on end but would require her to balance it with her fingers. Balancing will usually apply about 4 oz pressure on the fingers if done very casually and less than 2 oz if you put just a little effort into it.

Yes, just putting something on top of the scale and then zeroing it would be the most obvious most accurate solution.

That seems to introduce unnecessary additional risk of error. Surely it’s no trouble to flip the package around between the scale and the second support point?

Yes. In fact, I’ve lost 30 lbs. in the last month with this method!

No it would be more like putting you toes on one end and supporting your head on the other end and then reversing the process.