The Floating Needle

The Floating Needle

What will happen if you put a needle in a cup of water? It would sink. But you can do it if you use density.

Things you’ll need: a needle, a sharp pencil, A strip of paper that is a little bigger than the needle, a cup, and water.


  1. Fill water into the cup
  2. Put the piece of paper on the water.20170328_112911
  3. Put a needle on the piece of paper.20170328_112925
  4. Use the sharp point of the pencil to push the paper down to make it sink.20170328_112956
  5. Now, you should have a floating needle.

    Click on picture to make it bigger

A material’s density is defined as its mass per unit volume. It is, essentially, a measurement of how tightly matter is crammed together. The principle of density was discovered by the Greek scientist Archimedes. To calculate the density (usually represented by the Greek letter “ρ”) of an object, take the mass (m) and divide by the volume (v): ρ = m / v

The density of water is about 1 gram per cubic centimeter (62 lb/cu ft): this relationship was originally used to define the gram. The density varies with temperature, but not linearly: as the temperature increases, the density rises to a peak at 3.98 °C (39.16 °F) and then decrease. This unusual negative thermal expansion below 4 °C (39 °F) is also observed in molten silica. Regular, hexagonal ice is also less dense than liquid water—upon freezing, the density of water decreases by about 9%.

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