Rock and Boat: Density, Buoyancy & Archimedes’ Principle
Students are presented with a challenge question that they must answer with scientific and mathematical reasoning. The challenge question is: "You have a large rock on a boat that is floating in a pond. You throw the rock overboard and it sinks to the bottom of the pond. Does the water level in the pond rise, drop or remain the same?" Students observe Archimedes' principle in action in this model recreation of the challenge question when a toy boat is placed in a container of water and a rock is placed on the floating boat. Students use terminology learned in the classroom as well as critical thinking skills to derive equations needed to answer this question.
Archimedes' water balance shows that the laurel wreath is less dense than the pure gold block. The laurel wreath has a larger volume so it displaces more water and experiences a larger buoyant force.Copyright 2009 Tonyle, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Archimedes_water_balance.gif
Archimedes' principle and the density-buoyancy relationship are important in science, engineering and technology applications, such as the rise of a balloon in the air and apparent loss of weight of submerged objects and various floating vessels. Naval architects and engineers design the hull shapes of ships to be buoyant by distributing their weight over a larger surface area so the weight of the water displaced is greater than the ship's weight. Engineers must also know how much water ships displace in order to help them navigate through small channels and lock-systems. Engineers also apply their understanding of buoyancy and Archimedes' principle to design stable offshore oil rigs and production platforms.
After this activity, students should be able to:
- Answer the challenge question based on an understanding of volume, mass, density and weight.
- Explain why the water level rises/falls/stays the same based on an understanding of Archimedes' principle.