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## Class 9 Physics (India)

### Course: Class 9 Physics (India)>Unit 3

Lesson 7: Relative density

# Relative density (Specific gravity)

Let's explore what relative density is. Created by David SantoPietro and Mahesh Shenoy.

## Video transcript

what does specific gravity mean and just want to pause here and say specific gravity is also very often called a relative density okay that's it that's it let's continue the specific gravity of an object is the density of that object divided by the density of water the density of water is 1,000 kilograms per meter cubed for instance the density of gold is 19300 kilograms per meter cubed so the specific gravity of gold is 19.3 the density of ketchup is 1,400 kilograms per meter cubed so the specific gravity of ketchup is 1.4 note there's no units for specific gravity because it's the ratio of one density to another density so the units cancel each other out okay so why even bother defining something called the specific gravity well one really cool thing about specific gravity is that for something that floats the specific gravity tells you the fraction of that object that will be below the water while it's floating for instance say you let a cube of wood with specific gravity zero point to float in water since the specific gravity is zero point two that means that 20% of the total volume of this wood is gonna be submerged below the water while it's floating if the cube of wood had a specific gravity of zero point six sixty percent of the wood would be submerged beneath the water's surface ice has a density of about nine hundred and twenty kilograms per meter cubed that means ice has a specific gravity of 0.9 - and that's why 92% of an icebergs volume is actually underneath the water but what if we were to use a cube that had a density of 2700 kilograms per meter cubed the specific gravity would be two point seven which means that two hundred and seventy percent of this cube would be submerged beneath the water but you can't have more than a hundred percent of an object submerged even if the object were to sink the maximum amounts merged would be 100% so if the specific gravity of an object is greater than one that object is gonna sink if placed freely in water and it'll have exactly 100 percent of its volume submerged usually when people are referring to the specific gravity they're referring to the density of the object divided by the density of water but sometimes it's useful to define the specific gravity with respect to a liquid that's different from water for instance if you were to let some would of density 638 kilograms per meter cubed float in oil that has a density of 850 kilograms per meter cubed you could still find the percent of the wood that submerged underneath the oil just use the density of oil instead of the density of water in the specific gravity formula if we do that we'll find that the wood does flow in this oil with 75% of the woods volume submerged beneath the surface of the oil