I am having a problem understanding the concept of the apparent weight change when you crouch down, stand up, and jump on the scale. What is happening that makes the scale change?
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$\begingroup$ What makes that you go up and down? $\endgroup$– BernhardCommented Sep 22, 2015 at 17:31
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$\begingroup$ Welcome to Physics SE. Please look around and take the tour. If you understand how a scale works, than @Bernhard's comment should get you to an answer readily. $\endgroup$– Jon CusterCommented Sep 22, 2015 at 17:36
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$\begingroup$ I wonder who downvoted this question, and why. I think this is an excellent question. $\endgroup$– BernhardCommented Sep 22, 2015 at 17:39
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2$\begingroup$ When you read count_to_10's answer, remember that the scale doesn't measure your mass: It measures the force acting between the soles of your feet and the floor. $\endgroup$– Solomon SlowCommented Sep 22, 2015 at 17:56
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$\begingroup$ If it helps, consider what would happen if you pulled your feet up so quickly that your body fell freely toward the scale - that is, a REALLY fast crouch. During the time you were falling (the duration of this really fast crouch), what would the scale read? $\endgroup$– WhatRoughBeastCommented Sep 22, 2015 at 19:22
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1 Answer
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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.
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$\begingroup$ I miss the word acceleration here somewhere. $\endgroup$– pfnueselCommented Sep 22, 2015 at 18:03
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1$\begingroup$ That's patently false. 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. $\endgroup$ Commented Sep 22, 2015 at 20:34
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$\begingroup$ @CuriousOne the prodding was not intentional, but thank for you for the comment and the resulting giggle. I am not 100 percent convinced of the logic behind the source I used, I fully admit that and also that I definitely rushed the answer. Finally, from personal experience, you are perfectly correct regarding my suitability regarding experimental work, I have to date, mildly zapped myself electrically at least 10 times. $\endgroup$– user81619Commented Sep 22, 2015 at 23:50