Table of Contents Buoyancy


7. Ask Students, “What forces are affecting the object?” (There are two primary forces acting on it, gravity pushing down, and buoyancy pushing up.) Note: The gravitational force is determined by the object’s weight, and the buoyancy force is determined by the weight of the water displaced by the object when it is placed in water. If the gravitational force is less than the buoyancy force then the object floats (a boat), otherwise it sinks (a rock). That is, if an object weighs less than the amount of water it displaces then it floats otherwise it sinks.


8. Ask students to predict if the oil will sink or float and why. Check predictions by pouring 1 ounce of oil into it.


9. Explain to students that liquids of different densities (that don’t become a solution) separate into layers. Density is measured in kg/m³ meaning kilograms per cubic meter. Refer to the glossary for definitions of solution vs. mixture.


10. Pure water’s density is 1000kg/m³, therefore one cubic meter of pure water weighs one metric ton. Oil is usually around the 800kg/m³ area (as there are many oils which all have different densities) this means a cubic meter of this oil is 800 kg. Therefore the lighter less dense fluid floats to the top. Just like helium is lighter than air, a balloon full of helium would fly upwards, as oil does in water.


11. Sea water (salt water) is usually 1015kg/m³. Keep in mind, the density of fluid affects buoyancy of an object. For example, if a ship moves from sea water, salt water, to river water, fresh water, the ship will sink slightly. To allow the ship to continue to float, the weight of the water displaced by the ship must equal the weight of the ship. As the density of fresh water is less than sea water, the ship sinks a little further into the water to displace the extra 15kg per m³.


12. Mariners, especially those involved in cargo carrying, must calculate a fresh water allowance if they plan to go from sea water to fresh water to prevent the ship from becoming unstable in the fresh water. Discuss your local port. Do they have ships that travel from fresh water to sea water?


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