Surface Tension: How Many Pennies in Water Contest (Paul DeCunzo)

Standards

8a. Students know density is mass per unit volume.
solids and liquids) from measurements of mass and volume.
to the weight of the fluid the object has displaced.
- 8d. Students know how to predict whether an object will float or sink.

Questioning Script

Prior knowledge & experience:

Pennies will sink in water and take up space (have volume).

8b. Students know how to calculate the density of substances (regular and irregular

8c. Students know the buoyant force on an object in a fluid is an upward force equal

Root question/prediction/hypothesis:

How many pennies can I put into the beaker before it will overflow?
Students commit their # (in Quickwrite or on paper).
Ask students why they picked that # to reinforce "educated" guess.
What do we know about pennies in water?
Allow for changes to predicition.
I have students stand and as we add pennies, they sit down as their number gets past.

Expected Result : Stop at "bulge" in top of the beaker of water. What is that? What is causing that?

1. Density: Pennies (mostly zinc, D=7.95 g/cm3) Dwater=1.00 g/ml..
2. Surface Tension: the force holding the surface water molecules together is unbalanced at the surface. (Hydrogen bonding between water molecules).
3. Cohesion: force of attraction water molecules exert on each other.Water Molecules
4. Surfacants: substances added to water to reduce the surface tension.

Bridge Jumpers: Why do people jump from bridges to commit suicide. ie: The Golden Gate Bridge.

Cliff Divers: how do they survive?

Surfacants: how they work.

Washing Clothes
Back to the beacker of water: If we add soap will it take more or fewer pennies?
Add a drop of soap to the beaker and watch it "drain" down to the rim.

That water will spill over when above level to the rim of the container.

Water is easy to go into (slow speeds assumed).

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