Goal: Investigate water molecules and how they interact with each other to enable objects to both float and sink.

Time Required: 40 minutes

Materials Required: Why Do Boats Float? Student Reproducible 3 (PDF), pencil/pen, bowl of water, two sheets of aluminum (7.5 cm x 7.5 cm), a metric balance scale to measure weight

Directions:
1. Explain to students that there are several states of matter in which objects exist (gas, liquid, and solid). Almost all materials on earth exist in one of these states.

2. Write “water” on the board. Have students list any of the states of matter in which water may exist (all of them). Have students list characteristics of water for each of the states of matter in which it exists (solid—frozen into ice cubes, very cold; gas—moistened air, boiled into steam; liquid—natural state, wet). Explain that temperature plays a major role in determining of the state of matter (freezing point—32ºF, 0ºC; boiling point—211ºF, 100ºC; and natural liquid state—any temperature between freezing and boiling points).

3. Discuss how even though water can exist in any of three states, it is still water. Review the formula for density: Density = Mass / Volume. The amount of water in ratio to the amount of space it takes up is considered its density. The denser the object, the heavier and more tightly packed it is. Ask students, “Which do you think has a greater density, a pound of feathers or a pound of gold?” Have students imagine a pile of feathers and a pile gold, each weighing one pound, and have them compare the size of each pile. Ask, Which pile is larger? Why is the pile of feathers larger? (Answer: The pile of feathers is larger. Each individual feather is less dense than gold, so more feathers are needed to equal the same weight.)

4. Ask students in which states one could stay on top of water (solid—ice-skating, walking, fishing; liquid—boat, ships and people float on water). Ask students what would happen if a person waded into the deep end of a pool and curled up into a ball (person would sink). Ask students how many of them can float on the water and how they are able to do it (they spread out their arms and legs, arch their backs, and take a deep breath). Ask them if they know why. (Answer: Density is the mass of an object divided by its volume. If you increase the volume of an object, but keep the mass the same, the object's density decreases. We know that air is lighter than water. When you take a deep breath and hold it, you are increasing your volume, but your mass is only going up a little bit. In other words, you are adding air, which is very light, to your mass, so your average mass is going down. This has the effect of decreasing your density, making it easier to float.)

5. Distribute Why Do Boats Float? Student Reproducible 3 (PDF). Guide students through the experiment and have them record their findings.

6. Explain Archimedes’ Principle of Displacement and how the density of an object affects its ability to float or sink

• The principle states that the buoyant force exerted on a body is equal to the force exerted by the mass of the volume of displaced fluid (water). In other words, if a boat weighs 100 kilograms, it has to displace (push away) 100 kilograms of water in order to be buoyant (float).
• The volume of displaced fluid depends on both the shape and mass of the object.
• Remember that not only is the object pushing against the water, the water is pushing back! If the boat is pushing against a small area of water not equal to the weight of water pushing back, the boat will sink. On the other hand, since the boat’s weight is spread out over a larger area and there is more water to push back against the same weight, the boat will float.

Wrap-Up:
Have students draw or design various shapes of boats to determine if they are seaworthy.

Answer Key:
1) The ball sank; 2) The boat floated; 3) A) Both aluminum objects weighed the same, B) no,. C) yes.