Floating Coins
Not only can money talk, it can float too.

Concepts
Surface tension, density

Materials
2 Japanese Yen coins, a clear tall cup, water, paper clip
Procedure
  1. Bend the paper clip until it makes a shaft and triangular stand which can hold a Yen coin. See the picture below.
  2. With the Yen coin on the paper clip, gently lower it onto the water. Make observations of what happens.
  3. From a height of about 10 cm, drop the other Yen coin into the cup of water. Try to avoid hitting the other coin. Make observations of what happens.
floatingcoins1.jpg
Figure 46: A bent paperclip (left) can be used to hold a Yen coin (right) and gently lower it into a cup of water.


Results and Discussion
Answer the following questions:
  • Compare the Yen coins when they were initially placed on top of the water with a paper clip to when they were submerged in the water.
  • How does the density of a Yen coin compare to the water.
  • Make an inference to explain each Yen coins behavior in the water.
Explanation
Density can be defined as the amount of matter packed into a given amount of space. Mathematically, it can be calculated by dividing the mass by the volume. Water is often represented as having a density of 1.0 g/mL. Anything put in water that is denser than it will sink (greater than 1). Anything put in water that is less dense than it will float (less than 1).

Japanese Yen coins are made from pure aluminum, which has a density of 2.7 g/cm3. Given its density, the coins should sink when placed in water. Indeed this is the case if they are dropped in rapidly or placed under the water initially. Gently placing the Yen on the surface of the water enables the surface tension between the water molecules to hold the coins up. The attraction between like molecules is called cohesion and it is responsible for causing phenomena such as liquids forming into drops. The surface tension of the water can be broken by applying a force to the surface (such as poking the water) or by adding a surfactant such as dish soap.

floatingcoins2.jpg
Figure 47: Gently place a Yen coin on the water and it will float.

floatingcoins3.jpg
Figure 48: Drop a coin on the water and it will sink.


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