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Density basics 1. The density of an object will never change, no matter of how much or how little of that substance you have. 2. Because density does not change, it can be used to identify what a substance is made of. Density basics 3. Density means that you can have two objects that take up the same space, but have very different weights. For example, a 2-L bottle filled with soda would weigh about 5 pounds, where a 2-L bottle filled with gold would weigh 100 lbs! 4. Certain objects will float or sink in water, based on their density compared to water. 5. The density scale is based around water, which has a density of 1g/1 cm3. (you should already know that 1 cubic centimeter of water = 1 gram = 1 ml) Archimedes' principle is the law of buoyancy. It states that "any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body.“ The weight of an object acts downward, and the buoyant force provided by the displaced fluid acts upward. If these two forces are equal, the object floats. Density is defined as weight per volume. If the density of an object exceeds the density of water, the object will sink. http://www.onr.navy.mil/focus/blowballast/sub/work3.htm Archimedes' Principle If the weight of the water displaced is less than the weight of the object, the object will sink Otherwise the object will float, with the weight of the water displaced equal to the weight of the object. Archimedes' Principle explains why steel ships float Problem solving: Since we know that 1 cc of water has a mass of 1 gram, and we know that an object must weigh more than the water it displaces to sink, we can calculate whether an object will sink or float. • A 400 gram boat displaces 300 grams of water. Will it sink or float? ___________ Problem solving: • An object weighs 36 g in air and has a volume of 8.0 cm3. What will be its apparent weight when immersed in water? Solution: When immersed in water, the object is buoyed up by the mass of the water it displaces, which of course is the mass of 8 cm3 of water. Taking the density of water as unity, the upward (buoyancy) force is just 8 g. The apparent weight will be (36 g) – (8 g) = 28 g.