Penny Droplets
Materials:
o
An eye dropper
o
A penny
o
Liquid
dish soap
o
A
glass
o
Water
Part 1:
1. Guess how many drops you can
fit on
the front of the penny.
2. Try it. How many drops will stay on the face of the
penny without spilling water off the sides?
3. Repeat the experiment 3
times. The number of drops will probably
come out to be about the same each time.
Write down the numbers.
Part 2:
1. Add a tablespoon of dish
soap to your glass of water. Stir gently, don’t make too many bubbles.
2. How many soapy drops can you
fit on the front of the penny now?
3. 
Repeat 3 times and record
numbers.
Explanation:
You
just saw three important forces tugging on the water: gravity, cohesion, and
adhesion. Gravity flattens the droplets,
cohesion holds the droplets together, and adhesion holds the drops on the
surface of the coin.
The
cohesive force is the pull of the water molecules on themselves. Each successive drop sticks to the water
that’s already on the coin. = surface
tension
Soap
reduces the cohesive force, and break the surface tension. Less soapy drops fit on the coin.
Related Terms:
Gravity, Cohesion, Adhesion,
Surface Tension
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