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Name: Dylan J.
Status: student
Grade: 6-8
Location: WA 
Country: N/A
Date: 9/25/2005


Question:
Why do scientists need to know how dense something is? Would you please give real life examples that involve density?


Replies:
Density (weight / volume) allows one to convert between the volume of a substance and its mass (or weight). A very common "real life" example involving density is the fact that ice floats on water, because it is less dense than water. The principle is attributed to Archimedes. It states that an object is buoyed up by a force equal to the weight of the volume of liquid it displaces when placed in the liquid. So, for example, you can make a "boat" out of aluminum foil and float it on water, but if you "crumple" the same piece of aluminum foil it sinks when you place it in water.

Vince Calder


Density becomes important anytime someone wants to build something where weight and distribution of weight are critical. Ships require ballast to stay upright in the water, airplanes use counterweights to ensure they fly correctly. In either case, during the initial design, engineers must account for how much weight they need, and how much space must be allotted for it. To determine how much space they need, they must know the density of the materials they plan on using.

Ryan Belscamper


One of the places where density is important to me is in pipe design. Density is very important to me when I am trying to design a piping system to "push" a fluid though the pipes. In order to move a fluid through a pipe, I need a pump. I need to know certain properties of the fluid including viscosity (resistance to flow) and density in order to choose the right pump for the job. The density of the fluid I am using (as well as the viscosity) determines how big (i.e. how much horsepower) my pump will be. Think of it this way: the denser the fluid, the harder it is for me to push. Water is easy to push compared to your car's antifreeze (ethylene glycol), so I would need a bigger pump to move antifreeze. Yes, you have to design pipe systems to move antifreeze from one vat to the production line to put it into bottles for consumers. You even need to know the density of ketchup, mustard, mayonnaise, etc., in order to put it into bottles for consumers. Although I do not directly design systems for production like I just mentioned, I need to know the density of water and air at different temperatures to design the systems I use. We use water for different experiments with temperatures ranging from 40 F to 200 F, and air systems from 50 F to 1200 F. These things cause great ranges in the density of the fluid, so you can see how important density is to me.

Christopher Murphy, P.E.
Mechanical Engineer



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