Density and Floating ```Name: Ada Status: student Age: N/A Location: N/A Country: N/A Date: 2000-2001 ``` Question: Why do ice cubes float in water? Why doesn't the Ivory soap sink? Replies: Density. Ice is less dense than water and they add air to Ivory soap so it also is less dense and floats in water. Michael Baldwin When an object sinks into the water, the same volume of water has to be pushed up. If a quart of steel goes into a lake, a quart of water has to come up to the surface. It has to move out of the way. For most solid objects, this is no problem. Some materials, however, are not as heavy as water. A pound of ivory soap going into the water is only heavy enough to lift a pound of water out of the way. A pound of water is just as heavy as a pound of ivory soap, but it is not a large. Once a pound of water has been pushed up, the soap isn't heavy enough to push any deeper. The soap floats. Because a cup of soap weighs less than a cup of water, the soap is less "dense" than the water. Ice is less dense than water, too. These objects float. A cup of steel weighs more than a cup of water: steel is more dense than water. Steel does not float. Kenneth Mellendorf The upward force on an object in a fluid is equal to the weight of the fluid it displaces. If this force is greater than the downward force of gravity, it will push the object up until it reaches the point where gravity and the "buoyant" force are equal. Ice cubes float higher in salt water... it is more dense... than fresh water. Ice cubes sink in alcohol. Can you figure out why? You might want to read up on density and specific gravity. Larry Krengel Ice floats because ice has a lower density [specific gravity] than liquid water at temperatures equal to or less than 0 C. the melting point of ice. Ivory soap floats because it is, as advertised, "99.44% pure". The 0.56% is air intentionally whipped into the molten soap during the manufacture to give the overall product a density slightly less than water. The marketing ploy has served them well for decades! Although its only benefit is to help find the soap or to let babies make soap boats in the tub. Vince Calder Click here to return to the Physics Archives

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