What's the use of a penny? It's money that can float on the water.
What's the use of a penny? It's money that can float on the water.
Look for any more in your piggy bank.

Today, mobile phone payment is so convenient that even less cash is used to go out, and pennies have no opportunity to show their talents. But these worthless coins with little chance to spend are also precious experimental props: they can float on the water.

this is a very classic surface tension experiment: put the coin carefully flat on the water, it can be "held" by the surface of the water. This depends on the force of the surface tension. under the action of the surface tension, the water surface is like an elastic rubber film. The coin bends the local liquid surface downward, and the bent liquid film tends to contract, which creates an upward force to hold the coin.

(you can see that the water around the coin is bent)

however, not all coins can successfully complete the demonstration. There is also a limit to the support of the bending liquid to the coin. Although it is enough to support some small hydrophobic objects with a higher density than water, it will be difficult to succeed if it is too heavy. Among the coins I tried, the most successful ones were pennies (1, 2, or 5) and the previous version of the dime (I used the 2003 version, which looks like the picture, similar to the current version, but much lighter). These coins are much lighter than the dimes, dimes and one-dollar coins that are common on the market today.

of course, the method of placement is also critical: be sure to keep the coins flat and do not let the coins pass through the water. It can also be successful with bare hands, but the success rate is much higher with the help of gadgets. You can use a clip to make a small bracket, put the coin flat on it, and then slowly put it on the surface of the water.

how to make and use the bracket:

you can go through the change jar at home. If you still have these kinds of coins, take them out and play with them. Of course, even if there are no suitable coins, there are other things you can put, such as paper clips and pushpins.

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the pushpin is shaped a bit like a boat, but I can use the following picture to show that it is not enough to float by buoyancy alone. When cotton swabs dipped in detergent are immersed in the water around the thumbtacks, the local surface tension decreases and the thumbtacks sink:

A related phenomenon: why are bubbles always together?