Floating Needle


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If you proceed carefully and slowly enough, you will cause the paper to sink, leaving the needle floating on top of the water. The needle floats on what appears to be a film on the water's surface. When you touch the needle with the toothpick, you cause it to break through the surface of the water, and gravity causes it to sink quickly to the bottom of the glass.The needle floats because of cohesion and surface tension. The surface tension of the water at the top of the drinking glass forms an invisible film that is able to support the needle. Perform the experiment again after drying the needle thoroughly with a towel, but this time, look carefully at how the needle is resting on the water. You will observe that the surface of the water is bent under the weight of the needle!

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