Powered by soap
This is a
standard activity (probably any science for kids book/website
describes this activity) related to surface tension. It uses the fact
that soap reduces surface tension of water.
- Cut
a piece of index card or thin cardboard. Cut it so that it is tapered
on one side. Cut a notch from the other side (see picture below).
- Fill a large bowl or sink with water.
- Float the cardboard "boat" on the water surface.
- Add a drop of liquid soap/detergent just behind the notch and watch the boat move forward.
Explanation:
The soap reduces the surface tension. This causes the boat to move
forward. Drawing an analogy to a stretched surface could be useful.
Show your child a stretched rubber band and then cut it. Show them how
the two ends of the cut band pull away. In the same way, lowering the
surface tension is cutting the water surface. Now the rest of the water
surface (in particular the water in front of the boat) pulls away from
this point, pulling the boat along with it.
.
A
slightly modified version: I wondered what would happen if the soap was
attached and hence always behind the boat. So I attached a toothpick to
the boat using tape and then let the toothpick pierce through a small
bit of soap. The boat moved around for a few seconds before (I guess
the soap had contaminated most of the water surface).
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