How many more pennies can I add?
We begin with this simple activity which provides a nice visualization of surface tension.
- Fill
a glass with water all the way to the rim. Before you begin, make sure
that the glass is rinsed to remove any trace of detergent.
- Ask
your child to predict how many coins (pennies) they can drop into the
water without the water spilling over. If done with just a little bit
of care, they'd be surprised to see they can drop 15-20 pennies.
- Your
child shouldn't just drop the coin as it will only splash the water.
Instead bring the coin close and let it go when it is slightly immersed
in water (see picture).
Explanation: As more coins are dropped in, make
sure you draw your child's attention to the water surface. It should be
bulging out, sort of like a balloon. As you pump more air in the
balloon, its surface swells up. At some point in time, of course the balloon
will burst, letting the air out. Likewise, after more coins are
dropped in, the water surface "bursts" and a few drops are let out. But
water is like a self-healing balloon. When enough water is let out, the
surface is healed (until your drop the next coin).
Next surface tension activity
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