Wednesday, August 03, 2005

ICM - We Weigh More On a Carpet!

Today I had the first ICM session of the second semester in the Medical Building. It all started well and fine, and my group members consisted of 4 post-graduate students and 5 under-graduate students.

Since we are doing NDM this semester, our ICM naturally focuses on making nutritional assessment this time. For the first on-campus tutorial, we began with taking history of diets, activity level and anthropometric measurement (which is just another jargon meaning as simple as “measurement of the human body”).

The first two sessions were the similar routine, again we are familiarised with the basic interview strategies: open question first, focused question next and closed questions eventually. For example, what should we ask at the beginning? Well, basically the way we take dietary history was to have a record of what they call “24 hour recall”, so someone suggested that we might start off with “what did you eat for breakfast yesterday?” Our tutor Dr. Luke Dunne replied that it doesn’t make a good question. The reason? By asking what the patient ate for breakfast, we are already making an assumption that the patient did eat breakfast, which is not necessarily true. So instead of this opening statement, he suggested that we ask something which is wider in scope, e.g. “Can you tell me about what you ate yesterday?”

The first one and a half hour was basically about interviewing technique, so it was without any thrill or frill. Then we learnt the routine hand-wash technique, and it was not unfamiliar to me since I have learnt a bit when I was in St. John back in secondary school. However, I had to wash my hands for the second time before it was indeed germ-free under the examination of UV light. The first time there were quite a lot of remnant of germs which showed up like a sore thumb as florescent spots on my palm and the back of palms.

Then came the exciting part, the peer-to-peer measurement! Well, not too exciting, we are not going to measure anything “special”, just the height, weight, waist circumference and the hip circumference. My weight at the time being is 67.5 kg (down from 70kg a few months ago) and I am 1.75 metre tall–making my BMI 22.0 which is in the normal range. My waist-hip ratio (a ratio used to determine central obesity) is 0.8 which is again in the healthy range of male. Hey, I even qualified for the healthy range of female!! Muahahaha...

When we finally finished the ICM lesson, there came a heavy blow from my friend Xuan Ni–someone in their group told her that the reading of the bathroom scale is actually less than our actual weight! Oh gosh, that means I am actually more than 67.5 kg! Impossible!! I went straight back home and spent much time on research and discussion, and all I was thinking is.. how could that be? See, from the perspective of physics, the reading of a bathroom scale actually reflects the normal force of the scale to counteract our weight (which is caused by the gravitation force on our weight). So no matter what surface the scale is standing on, the scale would always need to exert a similar force to support me, or else I would have sunken to the floor!

Huhh.. after a little bit of Googling, I found some evidence that we indeed get different reading on different surface, but something that keep me relatively relieved is–we actually weigh more on a carpet! A lot of pages mentioned it, but I couldn’t really find the actual reason from the pages. Howstuffworks does have a page about the mechanism of the bathroom scale, but again there is no explanation to my pertinent issue.

There I was, searching and filtering one article after another... and there I was, a scientific studies and explanation about the issue in New Scientist! I couldn’t be more joyous than it, they proved that I actually weigh less than 67.5 kg now!! For a brief explanation, see the diagram below, sourced from the abovementioned article.

If the article and the picture is not enough, some eat-full-nothing-to-do scientist even did a thorough study on this phenomenon, and the pdf file of the research can be found here.

At the end, I can sleep well tonight. Good bye!


Winson Kang said...

What a nice post! i like it alot. I mean, it's basically all about physics rite? :-) But i have just one question, which may sound so damn easy for u but so damn hard for me. u know, i am still an amateur when it comes to physics. why horr, when the distance between the fulcrum and the loading point increases, the normal force exerted on us also increases ehh?

youngyew said...

Err.. the normal force that is exerted on us is still the same, only the force exerted on the spring changes. If you can see it in terms of torque, you could see that the total torque contributed by our weight is the same as the torque contributed by the tension force of the spring (although in the opposite direction).

So say that on a carpet (the base in which situation doesn’t bend), the person’s weight is 60g N and the distance to the fulcrum is 2cm. So if the distance between the fulcrum and the spring is 20cm, the force exerted by the spring on the lever would be 6g N. The spring mechanism on the scale would subsequently display the corresponding weight of 60kg on the plate.

Now move the scale to a hard surface, on which the base of the scale bends and therefore moves the fulcrum laterally. So let’s say the distance between the weight and the fulcrum decreased to 1.5cm. So now, applying the fact that total torque is zero, we can do the calculation.

From torque by weight = torque by tension force, we get 1.5 X 60g = 11.5 X tension force. And the answer would be 7.83g, which is larger than that of the previous case. Therefore, this extra tension force on the spring would translate to a larger reading on the scale.

Winson Kang said...

Err....actually i think of the easier way.Please help me verify whether it is true or not. In terms of torque, take the fulcrum as a fix point in each case. Well, as we know, the loading fulcrum ( the upper one ) transfer the weight of our body to the metal plate connected with the spring. Well, when the scale is on a carpet, the distance between the fulcrum ( not the loading fulcrum ) and the spring is larger compared to that when the scale is on the floor. Thus from the formula, torque= force multiply with the distance, since the distance between the fulcrum and the spring when the scale is on the floor, thus the torque exerted on the spring is larger, giving a larger value of weight. Am i correct?