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Name: Joanne
Status: educator
Age: N/A
Location: MA
Country: N/A
Date: N/A


Question:
Hi-

I work with Special Education children-grade 7 and 8- and there is a question that I cannot answer very clearly.... what are the differences and similarities between density and buoyancy? The text does not address this issue in a way that I can use to explain it to them so that they actually "get it". yet, it asks them to explain it.



Replies:
Hi, Joanne,

Have you talked about mass versus weight yet? That may help, though it may be hard to understand, also.

Examples may help. Density is a measure of how much mass something has, and so a denser object will weigh more than a less dense object of the same size. For example, a car battery weghs a lot more than a loaf of bread, yet a small car battery can be about the same size. Or you could use a wood or metal ball and a polystyrene foam ball the same size. Metal would carry over easily to the bouyancy example.

Bouyancy only happens when an object is immersed in a fluid. When something is immersed, a force acts on it that tends to make it float. The force is equal to the weight of the water displaced. So a big, light object (that is not too dense) will have a larger bouyant force acting on it than a small, heavy object (more dense). If the bouyant force is more than the objects weight, it floats.

At this point you could put the loaf of bread and/or battery into a bucket of water to see what happens, but at this point, the balls are definitely a better example. If you have all 3 types (wood, metal, and polystyrene foam), then you could show the difference in the height the wood and foam balls float at.

Hope that helps.

David Brandt, P.E.


I have a second answer/experiment that you can use to demonstrate density and buoyancy.

Obtain some plastic discs, spacers, or washers from a local hardware or building supply store. It does not matter too much what you use so long as they sink in water, do not dissolve or react with water, and are nearly identical to one another. Glass or plastic beads would also work and you can find them in most hobby shops. You can also string them on a thread or a fine monofilament fishing line to keep track of how many beads you have, in addition the thread or monofilament fishing line is light enough that you will not have to make any correction for the weight of the thread or fishing line. Using a lab balance/scale have the students graph the weight of the objects as a function of the number added to the balance/scale. The purpose of this is just to demonstrate that the weight (mass) is a linear function of the number of identical objects. Sounds pretty obvious, but still a good experiment. It also introduces the concept of "graphing" to represent numerical data.

Fill a graduated cylinder (say about a 100ml graduated cylinder) with water, and weigh the 100 ml of water. You will of course find that it weighs 100 gm because the DENSITY of water is 1 gm / 1 ml. You can repeat this step of the experiment with a soluble salt. Epsom salts (MgSO4*7H2O) is a good choice because it is readily available, cheap, and not toxic -- available at any pharmacy or grocery store. Make a concentrate solution (it need not be saturated, in fact it should not be). Weigh 100 ml of this solution. It will be heavier than the 100 ml of pure water. This demonstrates that solutions (liquids have different densities). Depending upon the type of balance/scale you have you may have to make a correction for the weight of the graduated cylinder (tare weight).

Now the fun begins. With the graduated cylinder about half full (say 50 ml) drop in the objects. You will notice that the volume as it appears in the graduated cylinder will increase. That increase is the volume occupied by the objects, it is the volume of water displaced by the beads. The weight of the objects measured in air divided by the volume of the objects is, by definition, its density.

Repeat the same submersion using the Epsom salts. The students will notice that the change in volume will be the same for both liquids. This shows that the volume of the discs, beads,... does not depend upon what liquid you use, so long as the objects do not float. The students may not have anticipated that the volume is the same no matter what the liquid phase is.

Now weigh the discs, beads,... submerged in water and compare that weight to the weight measured in air. The students should notice that the apparent decrease in weight of the submerged objects will be the same as the weight of the increased volume of water. So the bouyant force is the weight of the displaced water, which in the case of water is also the increase in the volume of water due to the immersion of the discs, beads,....

Now have the students repeat the weighing of the discs, beads,... submerged in the solution of Epsom salts. They will discover that the decrease in weight (bouyant force) is equal to the mass of the volume of solution displaced by the submerged beads. They will be able to deduce that because they have measured the density of the solution in the procedure described above for measuring the density of the Epsom salts solution.

You will need to do some preliminary work to find the best combination of objects and salt solution concentration etc. so that the "numbers" come out easily and the students will not get tangled up in the arithmetic manipulations.

Good Luck

Vince Calder


Density is a property of a substance. It is the mass divided by the volume of the material.

Buoyancy is how the densities of two substances (one usually a solid and the other usually a liquid) compare. If the liquid is less dense than the solid, the solid will sink in the liquid. If the liquid is more dense than the solid the solid will be buoyed up by the liquid.

Greg Bradburn



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