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Name: Sonya H.
Status: Other
Age: 30s
Location: N/A
Country: N/A
Date: 2002


Question:
My nine year old son and I are working on his science project. We have mixed three different types of liquids together to prove that one liquid is more dense than the other, and will separate. In other words, one liquid is heavier than the other.

I am having problem applying this concept to everyday life, on a level where he can understand. I would appreciate it, if you would give me some examples of why we need to know the density of a liquid, and how is it used in everyday life.



Replies:
I am an engineer who deals with fluid systems. To me density is very important when it comes to moving a fluid around. When I say fluid, I mean both liquids and gases. Imagine if I want to pump water from point A to point B. One of the important properties of the water is density, because it determines what size pump I need to move it down the pipeline. Now, imagine I want to pump ketchup from point A to point B. It is more dense, so I need a more powerful pump to get it down the line. Understand that density is just one of the properties that determines how easy it is to pump a fluid from one place to another. One example that your 9 year old would probably understand the best when it comes to density of gases is a helium filled balloon. Helium trapped in a balloon is less dense than the surrounding air and thus it rises. Hope this helped.

Chris Murphy PE


Wow!! Where to start. 1. Many items in the grocery store are packaged by volume, but sold by weight (potato chips for example). So are you getting ripped off by just buying the biggest bag? 2. The WEIGHT of any object floated on water is buoyed up by a force equal to the VOLUME of the fluid displaced (Archimede's Principle). So some objects sink in pure water but sink in salt water -- an egg for example. It's a matter of the density of the object (egg) and the fluid (salt water).

The conversion from one to the other involves the density. 3. Submarines float or sink depending upon how much their density is increases by taking on ocean water ballast. 4. All "shots" in a doctor's office are delivered by volume but are formulated by the weight of the active ingredients. The conversion between the two is density. 5. A back yard grill that uses propane as a fuel. The tank is filled with a certain weight of propane, but the temperature of the grill is determined by the relative volume of gaseous propane and air. Here we do not even do the calculation but still use the concept.

Any place where one needs to be able to inter-convert from the volume of something to its weight -- or the other way around -- uses the density to do the conversion.

Vince Calder



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