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Angela Morelli, age 12, of Utica, New York, for her question:

How did they weigh the world?

Weight is related to gravity and the strength of gravity can be measured. With the right apparatus, its force can be seen to exert a twist in a wire. Weight is related to balance and objects can be weighed on a pair of balancing scales. These known facts were used to construct a very complex instrument to weigh the whole world.

When scientists cope with planets, they refer to weight as mass. It means the amount of matter or material packed into a certain volume. Mass is related to gravity because gravity is a built in property of every particle of matter in the universe. The massive earth has a much larger quota of gravity than a speck of dust. But both quotas attract each other and obey all the other universal laws of gravity.

The earth exerts a tremendous pull on a stone and the stone exerts a tiny tug on the earth. If the mass of a stone is known, it is possible to calculate the tiny strength of its infinitesimally small quota of gravity. If it becomes a falling stone, it might be possible to estimate the strength of the earth's gravity from how fast the stone falls. The stronger the pull, the faster the fall.

However, there are handicaps to using this method to figure the earth's mass. For one thing, the air tends to make the stone lose speed. The job can be done more conveniently by comparing the earth's gravitational pull on a small sphere. The weight of the ball is its mass plus gravity and this must be measured precisely.

The apparatus used is called a torsion balance. Metal balls are carefully measured and suspended from the two ends of a horizontal rod. This is suspended from one very thin wire. To avoid air currents and other disturbances, the delicate apparatus is sealed.

A torsion balance measures the strength of a force by its exertion on the wire. The force causes a torque, or twist. The degree of the torque Is observed with the help of properly placed mirrors and telescopes. The torsion balance used to weigh the world, measured the degree of force exerted between the earth and the metal spheres.

The results made it possible to calculate the earth's mass from the gravity exerted on a small object. The final answer gave the weight of the world as six point¬six sextillion tons  which can be figured as 66 with a tail of 20 zeros.

For its size, ours is a dense and weighty little world. And it is gaining more mass or weight every day. Countless meteors continually crash through our atmosphere, burning to ashes as they descend. Tons of this meteoric dust filter down every year and add more weight to our already massive planet.

 

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