![]() (Unlike the Cheerio, however, the tack is not really floating. A tack can appear to float on water because its weight is counteracted by the surface tension of the water. The same principles also apply to denser objects. In both cases, the movements of the Cheerios are determined more by the geometry of the surface of the milk than by any attractive force acting between them. A second Cheerio placed into the bowl will form its own dent on the surface of the milk, and if the two Cheerios drift close enough to each other, they will appear to "fall into" one another, as if pulled together by an attractive force.Ĭheerios near the edge of the bowl float upwards along the curve of the meniscus to look like they're clinging the edge of the bowl. Place a single Cheerio in a bowl of milk and its weight will cause the milk beneath it to dip slightly, forming a dent in the once smooth surface of the milk. Viewed from the side, the surface of the water looks like a less exaggerated version of a skater's U-shaped halfpipe. It curves to form what scientists call a "meniscus." Depending on the properties of the liquid, whether it is attracted to or repelled by the glass, the meniscus either curves upward (concave) or downward (convex).įor water, the meniscus is concave since water is attracted to the glass. Water near the side of a glass behaves differently. So the surface of the water caves in slightly. Water molecules at the surface experience a strong inward pull from water molecules beneath them but only a weak outward pull from the air molecules above. Where it gets interesting is at the surface, where water meets air, and along the sides of the glass. But like two equally matched opponents in a game of tug-of-war, the net effect is zero and nothing really happens. In a glass of water, most of the water molecules are surrounded by other water molecules and they all pull on one another. ![]() It results from various weak forces acting between liquid molecules. Surface tension is a property that makes the surface of a liquid act like a flexible membrane. If an object is less dense than the water or air surrounding it, the object floats if the object is denser, it sinks. Buoyancy is what keeps ships afloat and balloons afloat. Buoyancy determines whether an object submerged in water or surrounded by air will sink, float, or stay put.
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