Welcome to You Ask Andy

Bernadette Doemer, age 13, of Fort Huachuca, Ariz..

What makes some objects float?

A plastic balloon filled with 875 cubic feet of helium gas can lift a weight of 35 tons into the air. A bar of iron sinks in water and floats in mercury The United Stages, our great ocean liner, displaces 53,000 tons of water and therefore weighs 53,000 tons. An iceberg displaces its own weight of water while one tenth of its volume floats above the surface A lump of ice, then, weighs nine tenths of an equal volume of water. A11 these facts concern the strange force of buoyancy ‑ the force which makes some things float and allows others to sink.

They principles of buoyancy ware discovered some 2200 years ago by Archimedes, a brilliant mathematician of ancient Greece. The story goes that he solved this mystery when he took a bath while cogitating a problem. of fraud. King Hiero of Syracuse suspected that a cheaper metal had been alloyed with the gold from which a crown was made. Archimedes was challenged to prove what if the crown was pure gold or an alloy of gold and silver ‑ without harming the, workmanship.

With this problem on his mind, he stepped into his bathing pool‑ and noticed two things. The water level rose slightly in the pool and his body seemed lighter in the water than in the air. He saw a relationship between the volume of water displaced and his apparent loss of body weight. The brilliant mind of this ancient scientist used thaw simple facts to work out specific gravity, the density of a substance in relation to water and the principles of the forts of buoyancy. We are told that ha used his new knowledge of specific gravity to prove that the king's crown was indeed alloyed with silver. His principles of buoyancy arcs used in designing balloons, aircraft, ships and any other objects which must float.

This principle of Archimedes states that a body immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced. This law applies also to gases, as shown in a lighter‑than‑air balloon. A lump of ice weighs only nine tenths as much as an equal volume of water: When in the water, it is buoyed up by a force equal to nine tenths of its total weight. This supports nine tenths of its bulk and the surplus weight floats above the water. Hence, nine tenths of an iceberg is submerged and only one tenth is above the surface.

Most of the solid objects on earth, however, weigh more than water. A stone weighs far more than the water it displaces and so dogs a bar of iron. However, iron weighs less than an equal volume of mercury and hence will float in this liquid metal. The volume of a big ship contains both heavy metals and light compartments of air. The total bulk weighs less than an equal volume of water, When at sea, it is buoyed up by a force equal to the weight of the water it displaces ‑ and part of the vessel floats above the surface.

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