See if you can answer the below question for yourself:
With both feet firm on a standard bathroom scale (no jumping) and without touching any other object (no pushing your hands against a low ceiling) is it possible to contort or move your body in a way that increases the force read on the scale to a value at least 5kg higher than your standing weight?
...
This question came to my mind as I was trying to understand how surfers generate speed via pumping. In the context of sports, pumping is the act of generating speed by contracting and extending one's body in accordance with gravity. Skateboarders pump to gain speed on a half pipe. Trapeze artists pump to swing like a pendulum. Children do the same on swings. The beauty of pumping is it's a way to generate momentum without having to push off of another object.
I correctly inferred the answer to my stated question is yes. But my intuition of how to apply the additional force to the scale was wrong. I thought the method was to stand up from a squat. My reasoning was the muscles in one's extending legs push off the ground, applying an extra force on the scale. In actuality standing up decreases one's apparent weight. And if you fully commit to the extension, i.e. by jumping, the force as read on the scale decreases to 0kg.
Since this misunderstanding is common, it is ~impossible to use search engines to find a proper explanation for the stated question. I found my broken logic marked as the correct answer to a related question on physics.stackexchange.com (archived.)
The question:
Why is the apparent weight smaller when you crouch down on a scale?
The question is asking for an explanation for something that is not true.
And the green check mark'd "correct" answer:
To shamelessly steal what James says above: the scale doesn't measure your mass, which remains the same no matter where you are, or what movements you make. The scale measures your weight, which is your mass multiplied by the acceleration due to the Earth's gravity, acting between your feet and the base of the scale.
You will measure your correct weight only if you stand on the scales without moving. As soon as you bend down, the muscles in your body that do the bending also act to pull up the lower half of your body. So this reduces the pressure your body places on the scales, and make you appear to weigh less.
Then, when you straighten up, your muscles act to force both the upper and lower halves of your body away from each other, now the scales will show a heavier weight since the lower half of your body puts a greater pressure on the scales.
Absolutely wrong.
There was a comment that appeared to come to the rescue.
That's patently false.
Yes it is! But the comment continues...
Your muscles don't have to pull any part of your body down. All you have to do is to put part of your body in free fall, in which case there is no necessity to hold it in place in an accelerated observer system like that of the floor. Try this by holding a 20lbs weight in both hands while standing on the scale, then let it go. Are your muscles performing any work? Is the scale showing 20lbs less while the weight is falling? Disclaimer: if you crush your feet while doing this experiment, I am not responsible, you are simply not suitable to be an experimentalist.
He fails to explain why the answer is wrong and appears to share the same misunderstanding of the author of the original answer.
Crouching down does not decrease your apparent weight. It's exactly opposite. Crouching temporarily increases the force applied to the scale. Straightening out temporarily decreases the force.1 To pump you squat while descending and stand up while ascending.
Answers to questions that require thinking are difficult/impossible to find through search engines. Google confirms what you already believe. This isn't restricted to politics. Google gives results that contain the wrong answer to basic questions regarding classical mechanics.
- The best explanation I can come up with is based on conservation of momentum. When one squats down they gain momentum downwards, so there must be something gaining equal momentum in the opposite direction. That something in this case is "the earth". When your body and the earth approach each other, your legs must exert an extra force on the earth to retard your momentum back to 0kgm/s as you come to a halt in the squat position. Then, when straightening out from a squat, your muscles extend to give you momentum in the upward direction. "The earth" simultaneously moves away from you, decreasing the force applied at the contact between you and the scale.
I don't have a scale with me in my apartment but I imagine if you put a scale sideways on the wall and lied down while crouched next to the scale and then pushed off the scale you would see a different result. Straightening out in this case would apply a force on the scale. I enjoyed working through this question so I'll leave the puzzle of figuring out why this scenario is different from the original vertical one as an exercise for the reader. [↩]
Just how many *more* instances of wasting time do you need in order to STOP searching for "the answer" and start aiming instead to *understand* what the question highlights? Because that's the very role of questions - to highlight something you don't yet understand, as a very first step towards gaining more knowledge, not more answers!
That aside, the title IS tedious and annoying in the extreme, indeed.
Dude holy god omfg why the fuck would you discuss this problem in terms of... fucking mommentum ?!
You understand it's a problem of acceleration, not of mommentum, why then are you in the wrong branch of the tree (appart from sub-par education in basic science, such as one would be exposed for instance at MIT, resulting in cluelessness as to what the fuck anything is or how it works).
There's two kinds of tools used in basic mechanics. On one hand the scalar tools : energy (mass * speed squared) ; mommentum / impulse (mass * speed), inertia (mass) etcetera. These are generally useful in conservative approaches. On the other hand the vectorial tools : velocity (distance/time IN A DIRECTION), acceleration (distance / time squared IN A DIRECTION), force etcetera.
As the guy points out : the scale does not measure mass ; it measures... force. Force.
I am aware that you read a number, and on this basis assume the scale to be in the scalar business ; but IT IS NOT. The scale's in the vector business, because of how it's made, which is what decides these things. Not "how it looks", that doesn't matter. How it works, that's what matters. The fact that it's not practical to add the unit of the involved vector space next to the value measured dun change its universally omnipotent omnipresence one whit.
Force (which is a vector) equals mass times acceleration (also a vector), thus therefore anything that changes your acceleration wrt the scale changes your perceived weight. Scales are adjusted to measure ~normal~ force ~at rest~, and so if you hit them they give off results.
Try it yourself : get a weight, place it on the scale. Then get the same weight, and THROW it on the scale. Or drop it on the scale from a height. Stand yourself on the scale, then jump in the air and LAND upon the scale at speed. Then get a spring, fix it vertically, measure how far it extends on its own weight, add an extra weight, measure how far it extends now, then LIFT THE WEIGHT AND LET IT GO, watch the spring go into a series of attenuated oscillations until stopping at the previously measured position. For half the time, the object "appeared heavier", though, didn't it, by the simple measure of having the spring extend below the neutral position.
Now try something else, like say place the scale vertically against the wall, and lie down next to it such that your feet are on the scale. It'll show something other than your weight, because you've violated its constructive expectations like you have before, though technically in a different way : this time, instead of using it other than at rest, you're using it to measure other than normal forces.
Because, again, it measures a fucking vector, and in order for the measurement to be meaningful, the presuppositions it relies on must hold : normal force (that is, force perpendicular on a defined surface) at rest.
Stop reading fucking webpages, go read Feynman if you're actually interested in physics.
@ Diana Coman
> Just how many *more* instances of wasting time do you need in order to STOP searching for "the answer" and start aiming instead to *understand* what the question highlights? Because that's the very role of questions - to highlight something you don't yet understand, as a very first step towards gaining more knowledge, not more answers!
>That aside, the title IS tedious and annoying in the extreme, indeed.
I need no more instances because digging around for an immediate answer via waddling through the swamps of search engine html is frustrating and useless, whereas reading a good source and working towards understanding is pleasurable and rewarding.
@ Mircea Popescu
> Dude holy god omfg why the fuck would you discuss this problem in terms of... fucking mommentum ?!
In retrospect it is certainly the wrong way to approach the problem.
> There's two kinds of tools used in basic mechanics. On one hand the scalar tools : energy (mass * speed squared) ; mommentum / impulse (mass * speed), inertia (mass) etcetera. These are generally useful in conservative approaches. On the other hand the vectorial tools : velocity (distance/time IN A DIRECTION), acceleration (distance / time squared IN A DIRECTION), force etcetera.
I was thinking of momentum in terms of a vector, I'm not sure why you listed momentum in the scalar category. (Although you define it as mass * speed whereas I was thinking of momentum as mass * velocity)
> I am aware that you read a number, and on this basis assume the scale to be in the scalar business ; but IT IS NOT. The scale's in the vector business, because of how it's made, which is what decides these things. Not "how it looks", that doesn't matter. How it works, that's what matters. The fact that it's not practical to add the unit of the involved vector space next to the value measured dun change its universally omnipotent omnipresence one whit.
I understand that force is a vector.
> As the guy points out : the scale does not measure mass ; it measures... force. Force. ..(and on)
I state in my question as "the force read on the scale" to make this point clear. I should have clarified that this question is meant to focus on "when is one increasing the normal force on a surface?" rather than how does a specific implementation of a scale perceive that increase.
As for the scale being placed vertically on the wall, I did not take into account that since the scale is likely a spring of some sorts, once it is placed on a wall gravity will bend it downwards slightly to take the form of a soft dick. That aside, from my understanding extending your body while lying on the floor and pressing your legs against the scale will increase the normal force on the scale because there is friction between your body and the ground that applies a force in the direction of the scale.