Explain Buoyancy and Floatation:
Materials whose densities are less than that of water have a tendency to be buoyant, or float on water. Buoyant materials include wood, oil, gases, and some materials like plastics. Usually, substances whose densities are greater than that of water will sink because the buoyant force pushing up on the object is less than the object’s or substance’s weight. Metal objects tend to fall into this category. Objects which have a very low average density - such as an air filled balloon - will have a high degree of buoyancy - or floatation - when placed in water. Other objects can float but will be partially submerged. In these cases, the buoyant forces are great enough to keep the object from sinking.
When we analyze the forces on an object floating in water, the weight of the water displaced by the object is equal to the buoyant force holding up the block. If we measure the displaced volume of water caused by the object, we can then determine the mass of the object. Using the relationships below, we can solve for mass.
Buoyancy = Density of Water * Volume of the Water Displaced by the Object * Gravity
Fb = ρw * V * g
Force due to Gravity = mass * object
Fg = m * g
Combined, this results in:
m * g = Pw * V * G and thus...
m = Pw * V or m = V
Because the floating object is not fully submerged, we do not know the volume. Either we can force the block under the water and measure the volume of water displaced, or we can try to calculate the volume if it is a regularly shaped object. Once we have the volume, we can find the density of the object.
*Instruct the students to open the laptops and select a browser.
*Instruct the students to go to the following URL for the density applet: Buoyancy-html version or Buoyancy-flash version
NB: Tips for teachers provided by PhET about the applet can be found at this link: Tips-html version or Tips-Flash version
Take the students through the program (Take as long as necessary). Show them how the amount of water displaced by a submerged object will be the same as the object’s volume (at least in water). Experiment with liquids other than water. The goal is for the students to familiarize themselves with the program and the concepts associated with it.
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