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?
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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. [↩]