Does the law of conservation of mass hold in the stomach?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

19 February 2006

A reader wants to know the effect of mining on Earth. Says he: “Since miners are constantly removing materials from the Earth’s crust, doesn’t this reduce the mass of the planet? Won’t this reduction eventually make the Earth so light that it will spin out of its orbit around the sun?”

True, if the mass of a planet is changed significantly, the planet will go out of its orbit around the sun. The key word here is “significantly”. The mass of the Earth is about 200 billion-billion tones, that is, the number two followed by 20 zeroes. A significant proportion of this might be, say, one per cent, which comes to 2 billion-billion tones. Has the Earth lost anything close to that since the beginning of modern civilization?

The answer is NO. Mining activities remove materials from inside the Earth and bring it to the surface. That doe not change the total mass of the planet since the mined material is still on the Earth. You don’t get poorer if you remove money from your pocket and hold it in your hand!

Even when something is mined and burned (like oil), it does not escape from the Earth. It stays in the atmosphere, which is still part of the planet. The only time the mass of the Earth changes is when an object is launched into outer space (but not in an orbit around the planet).

For example, when a space shuttle flies to the International Space Station, the total mass of the Earth does not change. Since the Station is in an orbit round the planet it is “seen” by the sun as part of the Earth. In fact, even the moon is a part of the Earth!

This case of mining brings to mind another interesting (but fallacious) question: Why doesn’t the law of conservation of mass hold in the stomach? That is, why is it that when you eat food, the mass of the food does not add to your body weight? The answer is that, when you eat something your body mass increases by the same quantity that you have consumed! If you don’t’ believe that, just try it!

It’s a simple experiment: weigh yourself, weigh the food, eat the food, and weigh yourself again. You will observe that your mass has increased by the exact same amount that you ate. If you think you’ll look stupid weighing yourself so many times, then think about it this way:

Suppose you take a plate of food and stand with it on a weighing scale. Assume you weigh 60kg, the plate 200 grams, and the food 300g. the scale would show 60.5kg. If you eat the food while standing on the scale, what would be observed?

If it were true that the food in your stomach does not add to the body weight, the scale reading would reduce as you consume the food. And when you finish eating, the final weight would be 60.2kg. Now that does not sound reasonable at all, does it?