A Moment of Science

The Floating Needle

Here's a neat experiment you can try at home!

Here’s a neat experiment you can try at home. All you need is a glass of water, a sewing needle, and a small slip of paper.

Try This!

Place the sewing needle on the little slip of paper and very gently rest the paper on the surface of the water. The paper will absorb water and soon sink to the bottom of the glass, but the needle itself will remain on top of the water after the paper has dropped away! How is this possible?

Well, you might say, the needle is lighter than the water, but even a slender needle has a much higher density than water.

Maybe the needle is displacing enough water to make it float, like a ship? Nope. If you look carefully you can see that the needle is right up at the surface.

Why Doesn’t It Sink?

The answer is in surface tension. Surface tension is the term physicists use to describe the tendency of the water molecules to adhere to each other where water meets air, so that they form a skin-like sheet. Thin as it is, this naturally-occurring skin is strong enough to hold the needle up and prevent it from sinking.

Then why did the paper sink? Well, remember that the surface tension sheet only exists at the border between air and water. The paper crossed over that border when it became soaked through with water. The needle, not being porous, remained dry.

What would happen if you wetted the same needle and tried again? As you probably figured, because it is already surrounded with water molecules, a wet needle will sink, just like water-logged paper.

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